Study of surface stoichiometry and luminescence efficiency of nearsurface quantumwells treated by hydrogen ions and atomic hydrogenYingLan Chang, Wolf Widdra, Sang I. Yi, James Merz, W. H. Weinberg, and Evelyn Hu Citation: Journal of Vacuum Science & Technology B 12, 2605 (1994); doi: 10.1116/1.587217 View online: http://dx.doi.org/10.1116/1.587217 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/12/4?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Longterm and thermal stability of hydrogen ionpassivated AlGaAs/GaAs nearsurface quantum wells J. Vac. Sci. Technol. B 13, 1801 (1995); 10.1116/1.587815 Study of hydrogenation on nearsurface strained and unstrained quantum wells J. Appl. Phys. 75, 3040 (1994); 10.1063/1.356150 Reduced quantum efficiency of a nearsurface quantum well J. Appl. Phys. 74, 5144 (1993); 10.1063/1.354276 Study of temperature dependent hydrogenation on nearsurface strained quantum wells J. Vac. Sci. Technol. B 11, 1702 (1993); 10.1116/1.586508 Luminescence efficiency of nearsurface quantum wells before and after iongun hydrogenation Appl. Phys. Lett. 62, 2697 (1993); 10.1063/1.109235

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Study of surface stoichiometry and luminescence efficiency of near­surface quantum wells treated by hydrogen ions and atomic hydrogen

Ying-Lan Chang, Wolf Widdra, Sang I. Vi, James Merz, W. H. Weinberg, and Evelyn Hu Center Fir QUllnti;:,ed Electronic Structures (QUEST). Unil'ersity of Calif(lrnia. Santa Barbara. California 93106

(Received 25 January 1994; accepted 19 April 1994)

A near-surface quantum well (QW) structure has been used as an effective probe of surface states before and after different hydrogen treatments. By correlating the surface composition with the luminescence efficiency of the near-surface QW, we find that the surface passivation is dominated by the defect density of the interface between the AIGaAs surface barrier and the overlying oxide. A complete recovery or further enhancement of luminescence can be readily achieved by treatments using hydrogen ions. However, atomic hydrogen at low exposures is not capable of modifying that interface, and is not effective in passivation. These results corroborate a passivation mechanism which involves removal of As from the interface between the AIGaAs surface barrier and the overlying oxide, and the reduction of interface state density.

I. INTRODUCTION

Effective passivation of surfaces can be a critical determi­nant of the electrical and optical performance of semiconduc­tor devices. This is particularly true for GaAs-based devices with a high surface recombination velocity and for low di­mensional structures with high surface-to-volume ratios. Re­cently, surface cleaning and passivation processes have re­ceived well-deserved attention in an effort to improve the surface/interface integrity in GaAs-based devices. While ex situ wet chemical I or photochemical2 cleaning and passiva­tion methods have appeared to reduce the surface-state den­sity, it is desirable to develop dry passivation and cleaning techniques for GaAs-based materials to improve the repro­ducibility and stability of the treatments.

The passivation of deep defects and shallow impurities in bulk semiconductors by hydrogen has been extensively studied.J.4 It has also been shown that a reduction of both carbon and oxygen on GaAs surfaces can be achieved by hydrogen plasma treatments.5~ 7 We have previously used the luminescence efficiency of a near-surface GaAs/AlGaAs quantum well (QW) as an effective probe of surface states.8

.9

and have found that hydrogen can cause a stable reduction of the nonradiative recombination centers on a native-oxide­covered AlGaAs surface. A complete recovery or even en~ hancement of luminescence efficiency was observed from a near-surface GaAs/ AIGaAs QW after low-energy ion-beam hydrogenation at room temperature.H

The effectiveness of hydrogen in the passivation of semi­conductor surfaces strongly depends on the materials and conditions of passivation. 10.1 I To offer insight into the chemi­cal changes of the surface layer during the reaction, we have utilized Auger electron spectroscopy (AES) to monitor, ill situ, the changes in composition of the surface that resulted from exposure to either hydrogen ions or atomic hydrogen. Ex situ photoluminescence (PL) measurements then provided an assessment of the passivation efficacy of the different hy­drogen treatments. The correlation between the surface stoichiometry and passivation efficacy should provide a bet­ter understanding of the passivation mechanism itself. Such

information is essential for process optimization and appli­cation to other systems and materials.

II. EXPERIMENT

The material structure used for this study is shown in Fig. I, and comprises a near-surface and a deep GaAs/Alo.)G<l(17As QW grown on a semi-insulating GaAs(IOO) substrate by molecular-beam epitaxy (MBE). All layers are non intentionally doped. The upper QW, denoted by Q I, is separated from the surface by a 350 A thick barrier layer (designated as a "surface-barrier layer" J, while the deeper QW, denoted by Q2, is separated by a 3500 A barrier layer from the upper QW. The deeply embedded QW is in­sensitive to surface states and can thus serve as a reference for normalization of luminescence intensity. The material was etched in a cltnc acidlhydrogen peroxide (C6H807/H202) solution at a volume ratio of 1:] for varying times, to produce different values of surface-barrier layer thickness d. The etch rate was calibrated using atomic force microscopy. 12

The samples were cleaned in a buffered HF solution, and then immediately introduced via an external load lock into an ultrahigh-vacuum (UHV) chamber with a base pressure of 5X 10- 11 Torr. Further details of the system are described elsewhere. 13 Briefly, the chamber is equipped with four-grid rear-view low-energy electron-diffraction (LEE D) optics, an Auger electron spectrometer with a single-pass cylindrical mirror energy analyzer, an ion sputter gun, a pin-hole gas doser, and a tungsten filament to produce atomic hydrogen. The chamber is pumped by a 1000 / /s turbomolecular pump.

Atomic hydrogen was generated by dissociation of mo­lecular hydrogen with a hot (1800 K) spiral filament posi­tioned approximately 5 cm within line of sight of the sample. The exposures given here were calibrated using atomic hy­drogen adsorption on a clean Si(lOO)-(2X 1) surface, and as­suming an initial adsorption probability of unity for atomic hydrogen on this silicon surface. 14

Hydrogen ions were generated by using an ion sputter gun

2605 J. Vac. Sci. Technol. B 12(4), Jul/Aug 1994 0734-211 X/94/12( 4)12605/5/$1.00 ©1994 American Vacuum Society 2605

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2606 Chang et al.: Study of surface stoichiometry and luminescence efficiency 2606

AIGaAs 350 A

GaAs 60 A

AlGa As 3500 A

GaAs 100 A

AIGaAs 350 A

GaAs substrate

d

Ql •

Q2

FI(;. I. Sample structures. The A I fractional compo,ition is 0.3. The upper QW is denoted hy Q I. and the deeper QW is denoted hy Q2. The surface­harrier layer thicknc" is denoted hy d. and is 350 A for the as-grown sample.

with differential gas inlet, operated at an ion energy of 100 eY. The flux was measured using the ion current collected on the sample. A typical ion current density of 1.5 f.LAlcm2 was used.

For the AES measurements, a PHIlO-ISS cylindricai mir­ror analyzer was used with an initial electron energy of 3 keY. The Auger peaks used for the identification of 0, Ga, As, and AI were those of energy 502, 1070, 1228, and J 396 eV, respectively. To minimize the diffusion of hydrogen dur­ing the hydrogen treatments and to avoid radiative sample heating in front of the hot filament, the samples were cooled to 200 K.

The luminescence efficiency of the near-surface QW waf; determined by ex situ PL measurements. The near-surface QW sample was held at a temperature of 1.4 K, and was excited by a Ti-~apphire laser with a power density of ~ 1 W/cm 2

. The PL spectra were taken with a GaAs photomul­tiplier detector. The excitation wavelength was set to 7540 A. [n this case, the excitation energy is below the band gap of the AIGaAs barrier material, so that excitons can only be generated in the quantum well region.

III. RESULTS

A. In situ AES measurements

AES was used to provide information on the composition of the native-ox ide-covered AIGaAs surface before and after the different hydrogen treatments. The Auger peaks used for the identification of Ga, As, and AI were those of energy 1070, 1228, and 1396 eV, respectively. The approximate es­cape depth of these Auger electrons is ~25 A.15 We have not measured the thickness of the native-oxide layers on our samples, but previous reports have given the thickness of the

J. Vac. Sci. Technol. B, Vol. 12, No.4, Jul/Aug 1994

7

6 '" .~ .... ell 5 '"

(ii) .... ,.0::: Oil 4 'a;i

,t:l

,.lCl 3 CIS ~ ~

r.n 2 ~ < (i)

1

Hydrogen ion exposure (xlOI6cm·2)

FIG. 2. The variation of Ii) the As(l228 eVj/Ga( 1070 eVj AES peak height ratio and (iij the 0(502 eV)lGa(l070 eV) AES peak height ratio with hy­drogen ion exposure. The solid curves are guides for the eyes.

GaAs native oxide as ranging from 5 to 10 A.7 Even allow­ing for a slightly thicker native-oxide layer on the AIGaAs surface, we expect that we are probing the thin native oxide, as well as the uppermost AIGaAs surface. This constitutes our "surface," for subsequent discussions, that is, native­oxide layer, the uppermost AIGaAs surface, and the interface between the two. The relative magnitudes of the peak heights for different compositions in AES spectra were found to be reproducible. The As/Ga peak height ratio for the unhydro­genated sample was measured to be ~ 1.05. Note that this simple peak height ratio is not an indication of the surface stoichiometry, but only serves to normalize the data for the hydrogenated samples.

Figure 2 shows the variation of the AES peak height ratios for the native-ox ide-covered AIGaAs surface with hydrogen ion exposure. As can be seen from the experimental results, low exposures (~I 0 16 cm -2) of hydrogen ions produce a modified, more Ga-rich surface, with no significant change in the oxygen concentration. The departure from the initial As/Ga peak height ratio increases with the ion exposure. At an exposure of 5X 1016 cm- 2

, the As/Ga peak height ratio is reduced to ~0.65, whereas the OlGa peak height ratio fluc­tuates slightly, but remains fairly high at ~6.2.

However, contrary to the results of the hydrogen ion treat­ment, low exposures of atomic hydrogen do not reduce the As/Ga peak height ratio, which remains approximately the

1017 -2 same for atomic hydrogen exposures of up to ~ cm, as shown in Fig. 3. Only at much higher exposures of atomic hydrogen (~5 X 10 17 cm -2) does this ratio significantly de­crease. At those higher exposures, the oxygen concentration of the near-surface region is also strongly reduced. For clar­ity, the comparison of As/Ga peak height ratios for samples having similar exposures of hydrogen ions and atomic hy­drogen is shown in Fig. 4.

No significant change of the AI/Ga peak height ratio was

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2607 Chang et sl.: Study of surface stoichiometry and luminescence efficiency 2607

7

6 .g -eo 5 "'" -.c: tlJ) 4 .... ;,I .c: ~ 3 eo ;,I Q..

CI:l 2 \:JJ (i) -<

1

0 0 10 20 30 40 50 60

Atomic hydrogen exposure (xlO16cm·2)

FIG. 3. The variation of (i) the As(1228 eV)/Ga(1070 eV) AES peak height ratio and (ii) the 0(502 eV)fGa(I070 eV) AES peak height ratio with atomic hydrogen exposure. The solid curves are guides for the eyes.

observed after exposure to either hydrogen ions or atomic hydrogen. We have also observed that low exposures of hy­drogen ions efficiently removed carbon contamination from the surface.

After both hydrogen ion and atomic hydrogen treatments, and before transfer out of the UHV chamber, the samples were allowed to warm up to room temperature. In all cases, no significant differences in the AES peak heights were found.

'" .~ 1.2 .... = 10.

i 'Q:j 1 -= ~ 8. 0.8

= C!I ~

-< 0.6

(ii)

(i)

2 4 6 8 10

Hydrogen exposure ( xlOH cm·2)

FIG. 4. The variation of the As(l228 eV)/Ga(1070 eV) AES peak height ratio with similar exposures of (i) hydrogen ions and Oi) atomic hydrogen. The solid curves are guides for the eyes.

JVST 8 - Microelectronics and Nanometer Structures

TABLE 1. Hydrogenation conditions.

Treatment

Hydrogen ion Hydrogen ion Atomic hydrogen Atomic hydrogen

Exposure (cm-z)

1016

5x 1016

4x 1015

2X10 11>

B. Ex situ PL measurements

AslGa peak height ratio

0.76 0.65 1.03 1.02

The variation of luminescence efficiency after the differ­ent surface treatments was determined by ex situ PL mea­surements. Our experimental data show a clear reduction in luminescence efficiency of the upper QW with decrease of the surface-barrier layer, consistent with our previous report.8 The normalized luminescence efficiency (1/), defined as (IQIIIQ2)dl(lQIIIQ2)3S·grown, is only ~0.45 for d=80 A. This structure, with strongly reduced luminescence effi­ciency without passivation, was then used to study the pas­sivation achieved after different hydrogen treatments.

The hydrogenation conditions used for the various samples, and the corresponding As/Ga peak height ratios, are summarized in Table I. In these cases, the OlGa peak height ratios were unchanged after both hydrogen ion and atomic hydrogen treatments. We use these data to infer that the native-oxide layer was largely unchanged during the hydro­genation process, and therefore we expect no significant changes in the surface composition to take place during the transfer necessary for ex situ PL measurements. However, since the As/Ga peak height ratio shows significant differ­ences from the unhydrogenated samples, we can examine possible correlation of that change in the surface composi­tion with the relative change in luminescence efficiency of the near-surface QW brought about through the different hy­drogen treatments.

The variation of 1/ is summarized in Fig. 5. As shown in Fig. 5(c), a complete recovery of luminescence efficiency was observed after hydrogen ion treatment with an exposure of 1016 cm- 2

• For a hydrogen ion exposure of 5X 1016 cm-2,

Fig. 5(d), 1/ is greater than unity. However, after exposures to atomic hydrogen of either 4XI0 15 or 2XI0 16 cm-2

, Fig . 5(e), no change of 1/ was found, nor was there any change of the As/Ga peak height ratio, as observed by AES.

Figure 6 shows the PL spectra for samples treated by dif­ferent exposures of hydrogen ions. Curve (a) of Fig. 6 dis­plays the PL spectrum obtained from an as-grown sample, where the higher energy peak denoted as Q I at ~ 1.648 e V is from the near-surface QW and the other one denoted as Q2 at ~ 1.59 e V is from the deep QW. Each excitonic peak corre­sponds to the fundamental transition between electron and hole states (e I-hhl). The PL spectrum for the untreated sample with a reduced barrier thickness of 80 A is shown in curve (b). For a similar sample, a complete recovery of the luminescence efficiency for Q I was observed after an expo­sure to 1016 cm -2 hydrogen ions, as shown in curve (c). An increased exposure of 5 X 1016 cm -2 produced a broadening of Q I, accompanied by a ~6 me V redshift, relative to the energy peak position of the as-grown sample. Both the peak

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2608 Chang et al.: Study of surface stoichiometry and luminescence efficiency 2608

» 1.6 CJ

= .~ CJ

E 1.4

~ 1.2

~ CJ

= 1 t ~ CJ rIJ ~

= .- 0.8 S ,E 0.6 "Q f ~ 0.4 .~ '"; S 0.2 "'" 0

;Z 0 (a) (b) (c) (d) (e)

FIG. 5. The variation of normalized luminescence efficiency. (lQIIlQl)JI(/QII/Q2)"··,,,·~"n. of the near-surface QW before and after different surface treatments, (a) as-grown (<1=350 A); (b) as-etched (<1=80 A); Ic) hydrogen ion exposure of IO lh em 2 (d= 80 A); (d) hydrogen ion exposure of 5X 1010 em 2 (d= 80 A): and (e) atomic hydrogen exposures of 4x 1015

or 2Xl0 Io cm- 2 (d=80 A)

in intensity and the energy position of Q2 remain the same as the as-grown sample.

IV. DISCUSSION

The use of a near-surface QW probe, and the choice of PL excitation energy below the band gap of the barrier were dictated by the desire to examine the effects of hydrogen treatment at the surface of the materiaL rather than in the

» -'til = ~ -= .-

~r~~'~~' I ,t _~~~_~.

1--<'<') ..../\. .~~I

1.58 1.6 1.62 1.64 1.66 1.68

Energy (eV)

FIt;, 6, The PL spectra before and after hydrogen ion treatment. (a) as-grown Id=350 A); (b) as-etched (ti=80 A). (e) hydrogen ion exposure of 1010

cm 2 Id=RO A): and (d) hydrogen ion exposure of 5X!OI6 em- 2 (d=80

A)

J. Vac. Sci. Techno!. e, Vol. 12, No.4, Jul/Aug 1994

bulk. It is important to verify whether this is indeed likely. and whether the recovery and enhancement of luminescence can be identified with hydrogen passivation of the surface. We have chosen an excitation energy so that carriers are generated only within the QW. For quantum wells separated from the surface by a barrier thickness d= 350 A (or larger), we assume that excited carriers will either recombine radia­tively with a lifetime Tr or undergo nonradiative recombina­tion with defects within the QW itself, or near to the GaAsl AlGaAs interface. This latter process has a characteristic lifetime Tnr' Therefore we can express the quantum effi­ciency of that quantum well as (J/rr)/( IITr + IITnJ This value was estimated to be -0.6, by excitation density-dependent time-resolved PL measurements,16 allowing us to deduce that Tnr is approximately 1,5 times larger than Tr in this case. For a near-surface QW with barrier thickness d= 80 A, the lu­minescence intensity is reduced, suggesting that an addi­tional nonradiative recombination path has been opened up, dependent on the proximity of the surface. Using a suffi­ciently low measurement temperature, and excitation power, we can neglect the thermionic emission of the generated car­riers from the well, over the barrier and to the surface; we will assume that the additional pathway involves tunneling from the QW to surface. The normalized luminescence effi­ciency of this QW can then be expressed as 17=(l/Tr+I/Tnr)/(1/Tr+I/Tnr+llrr); the value of 17 is only -0.45 before hydrogenation. Therefore, the tunneling time (1() can be estimated to be -0.497r . If the primary effect of the hydrogen was the passivation of defects in the QW alone, affecting only Tnr , the maximum 17 achievable after the hy­drogen treatments would only be ~0.55. However, as shown in Fig. 5, 17 was -1.0 and -1.3 for hydrogen ion exposures of 1016 and 5XI016 cm- 2

, respectively, Passivation of the bulk is not sufficient to explain the magnitude of the lumi­nescence enhancement observed; passivation of the surface must play a major role in the enhancement observed.

Our data show no significant change of 17 after treatments in atomic hydrogen for exposures as high as 2X 1016 cm- 2

•

Furthermore, the AES measurements reveal a reduction of the As/Ga peak height ratio for the hydrogen ion treatments, which does not apply for the atomic hydrogen treatments. The correlation between efficacy of surface passivation and variation of surface composition, as determined by AES, leads to the following conjecture: The loss of the surface As must be understood as the result of reactions between the substrate and the hydrogen ions, yielding volatile arsine (AsH3) and unsaturated surface/interface Ga atoms,17 The passivation mechanism may involve the removal of As. The reduction of interface state density can be understood in terms of the models proposed for the pinning of the Fermi level (see for example, the advanced unified defect model 18

and the effective work function modeI 19). The Ga-oxide and/or AI-oxide layer serves as a passivation overIayer for the As-deficient interface, and is responsible for the long­term stability of the passivation effect. Such a mechanism has already been proposed to explain the passivation brought about by hydrogen treatments on an oxide-contaminated GaAs surface.20,21 The data also suggest that the lumines­cence efficiency of the near-surface QW is dominated by the

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2609 Chang et sl.: Study of surface stoichiometry and luminescence efficiency 2609

defect density at the interface between the AlGaAs surface barrier and the overlying oxide, rather than by modifications on the surface of the oxide itself. Thus, hydrogen ions pro­duce increased luminescence for the near-surface QW, while not appearing to alter the OlGa peak height ratio at the sur­face. On the other hand, atomic hydrogen at low exposures is not capable of modifying that interface (As/Ga peak height ratio remains -I). Although the OlGa peak height ratio is reduced at high exposures of atomic hydrogen, this treatment is nevertheless not effective in the passivation of a native­oxide-covered surface.

We note that treatment of the surface by hydrogen ions at high exposure (5X 1016 cm-2

) gives a luminescence effi­ciency greater than that of the starting material. In this case, passivation of both surface states and defects in the QW itself was achieved. The observation of broadening effect and redshift of Q1, as shown in Fig. 6(d), may be due to interac­tion between surfacelinterface states and QW eigenstates, which still requires further investigation.

v. CONCLUSIONS

In summary, we report in situ monitoring of the compo­sitional changes of a nati ve-oxide-covered AlGaAs surface treated by both hydrogen ions and atomic hydrogen. The luminescence efficiency of a near-surface QW was used as an effective probe of surface states before and after different hydrogen treatments. We find that the passivation mechanism of the hydrogen ion treatments involves the removal of As, and therefore, the reduction of defect density from the native-oxide-AlGaAs interface. This has been determined through correlation between AES and PL measurements. The comparison of similar exposures of 100 e V hydrogen ions and atomic hydrogen clearly shows that the hydrogen ion treatments are able to modify the oxide-AlGaAs interface even at low exposures, whereas atomic hydrogen exposures at temperatures of 200-300 K seem not to affect that inter­face. However, at much higher exposures, atomic hydrogen treatments do reduce the surface oxygen, which may ulti­mately produce a change at the oxide-AlGaAs interface as well. This study shows that PL measurements of a near­surface QW, correlated with surface analysis, constitute a powerful combination for further optimization and under­standing of passivation processes. Further work is going

JVST B - Microelectronics and Nanometer Structures

on to better understand the details of the hydrogenation process through the native-oxide layer and at the oxidel semiconductor interface.

ACKNOWLEDGMENTS

The authors are grateful to Dr. I-Hsing Tan for helpful suggestions and stimulating discussions. This work was sup­ported by the NSF Science and Technology Center for Quan­tized Electronic Structures (QUEST), Grant No. DMR 91-20007, the W. M. Keck Foundation, and a MICROISBRC grant. One of us (W.w.) thanks the Alexander-von-Humboldt Foundation for a Feodor-Lynen Research Fellowship.

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