Sub-picosecond intersub-band electron scattering times in GaN/AIGaN superlattices grown by molecular beam epitaxy

H.M.Ng, C.Gmachl, S.V.Frolov, S.N.G.Chu and A.Y.Cho

Abstract: GaN quantum wells were studied, grown by plasma-assisted molecular beam epitaxy with a sub-band spacing of - 740 meV (A = 1.67 pm). The GaN quantum wells are clad on both sides with short-period superlattice barriers. Using the time-resolved pump-and-probe technique, with 1.55 pm pump and 1.70 pm probe wavelength, an intersub-band electron scattering time of 370 fs was measured.

1 Introduction

There has been a growing interest in intersub-band (IS) optical transitions at communications wavelengths (A - 1.55 pm). This interest is partly stimulated by the need to develop faster optoelectronic components, in order to keep up with the increasing demand for a higher bandwidth in optical communications. Optoelectronic devices based on the intrinsically fast IS transitions may be able to satisfy this demand. The material systems that can support IS transitions at such short wavelengths are quickly maturing.

Iizuka and Suzuki [1] have reported IS transitions as short as 2.8 ym peak wavelength using the GaN/AlGaN system. Iizuka et a/ . [2] have reported a measurement of the IS scattering time of 150 fs for an IS transition wavelength of 4.5 pm. Using Sb-based heterostructures, Mozume et al. [3] reported similar measurements for wavelengths around 1.55 ym and with - 800 fs IS scatter- ing time. Akiyama et al. reported a recovery time of 685 fs at 1.55 pm in coupled InGaAs/AlAs/AlAsSb quantum wells [4]. Using a similar structure, Yoshida et al. demon- strated all-optical switching using 1.55 pm IS transition with 1.3 ps response time [5]. Other material systems for which short wavelength IS transitions have been reported are InGaAs/AlAs [6] and ZnSe/BeTe [7].

We have reported IS absorption measurements in GaN/AlGaN multiple quantum wells (MQWs) at peak wavelengths as short as 1.41 pm [8], which lie in the important communications wavelength range. In this paper, we discuss the growth aspects and present the

0 IEE, 2001 IEE Proceedings online no. 20010752 Dol: 10.1049/ip-opt:20010752 Paper first receivcd 17th May and in revised form 28th September 2001 H.M. Ng, C. Gmachl, S.V. Frolov and A.Y. Cho are with Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill, NJ 07974, USA S.N.G. Chu is currently with Agere Systems, 600 Mountain Avenue, Murray Hill, NJ 07974, USA

IEE Proc.-Optoelectvon., Vol. 148, No 516, OctoberlDeceniber 2001

measurement of the electron scattering time for these IS transitions. Using a pump-and-probe technique, with 1.55 pm pump and 1.70 ym probe wavelength, respec- tively, we measure an intersub-band scattering time of 370 fs.

Straightforward calculations of the scattering time invoking only the emission of longitudinal optical (LO) phonons [9] result in the expected scattering time of - 1 10 fs; the discrepancy with the measurement is attrib- uted to the simplicity of the model and the limited knowl- edge of some material parameters [2]. The longer scattering time as compared to that of lizuka et al. [2] is explained by the much larger intersub-band energy spacing: 740 meV (2 - 1.67 pm) against 275 meV (1 - 4.5 ym [2]) and the need to exchange a much larger k-vector phonon. The shorter scattering time compared to that of Mozume et a/ . [3] is similarly explained by the larger LO phonon energy of the GaN-against the GaSb- based materials: - 90 meV against - 40 meV [3].

2 MBEgrowth

The biggest challenge for growing high-quality 111-nitride epilayers remains the unavailability of a native substrate with a low dislocation density. In our experiments, we used (0001) sapphire substrates coated on the underside with 300nm of Ti to improve the heating uniformity. Before introducing the sapphire substrate into the load-lock of the molecular beam epitaxy (MBE) chamber, the substrate was cleaned using a 10 : 1 H2S04 : H202 solution, rinsed with deionised water and dried using a spin-drier. Active nitro- gen was generated by passing high-purity molecular nitrogen through a radio-frequency plasma source. The group-111 species (Ga and Al) and the n-type dopant (Si) were evaporated from standard effusion cells.

We routinely start the growth with a high-'temperature A1N buffer layer, which is grown to a thickness of about 20-3Onm, followed by the growth of the GaN epilayer, both at a substrate temperature of about 720°C. The substrate temperature was monitored using a thermocouple located behind the substrate, as well as an infra-red

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pyrometer. Owing to the large lattice mismatch between (A1)GaN and sapphire, several threading dislocations are formed in the epilayer. In the worst-case scenarios, samples have been reported to have dislocation densities of up to 101o--lO'l cmp2 [lo]. To reduce the dislocation density, we have employed low-temperature AlN layers (- 20 nm) grown at 425"C, spaced 100 to 200nm between high- temperature GaN layers.

Fig. 1 shows a cross-sectional transmission electron microscope (TEM) image of a GaN epilayer, grown with two additional low-temperature AIN layers (LT-A1N) [ 1 13. Under this imaging condition, only dislocations with a screw component are visible. The screw and mixed dislo- cations were found to be reduced from - 2 x IO9 cm-' in the GaN grown after the first AIN nucleation layer, to - 5 x 10' cmP2 after 0.6 ym of GaN film grown following the LT-AlN layers. The edge-type dislocations were reduced by about a factor of two from 6 x lo9 cm-* to 3 x io9 cmP2.

We have grown samples with up to three low-tempera- ture AIN layers and, by examining the cross-sectional TEM images, we found that there is minimal additional reduc- tion in the threading dislocation density after the second A1N layer. This may be due to the lower probability of two dislocations with opposite Burgers vectors interacting with each other and annihilating, since they are spaced further apart after the initial LT-A1N layers.

The sample for the optical studies consisted of 15 GaN quantum yells (QWs), each 6 monolayers (ML; 1 ML-2.6 A thick which were doped with Si to n - 1 x 10' cm . This resulted in the Fermi energy of - 180 meV The barriers between the QWs were, in fact, superlattices of three 3 ML-wide GaN QWs, interleaved with 6 ML-wide A10 6sGao 35N barriers. This configuration allows the use of lower A1N mole fraction barriers, while still retaining a large confinement potential provided by electron Bragg reflection from the superlattice [SI.

It is advantageous to be able to obtain information on the growth rate of both GaN and A1,Gal -xN in situ, in order to achieve the targeted layer thicknesses. We used pyrometric interferometry to monitor the growth rate during the growth of the thick GaN layer underneath the MQWs [12]. Using the growth rate information extracted in real- time, we were able to calculate the growth duration required for the various layers in the MQWs. The accuracy of this technique has been cross-checked by performing post-growth analysis.

.

2 i 3 , . '

230 nm

Fig. 1 Cross-sectional transmission electron microscope image showing reduction of threading dislocation density after deposition of two low- temprrature AlN luyers indicated by arrows screw and mixed dislocations are visible under this imaging condition [ l 11

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Fig. 2 Cross-sectional transmission electron microscope image of portion of GaN MQW sample with superlattice barriers

GaN A10 &a0 35N P I

Fig. 2 shows a cross-sectional TEM image of the structure [SI. All the individual GaN and AlGaN layers are resolved, indicating that there are abrupt interfaces between the layers. The entire MQW region (including the superlattice barriers) was grown continuously, without any growth interruptions, at a growth rate of about 0.2 pm/h. We have also grown MQW samples with growth rates of between 0.4-0.5 pm/h, and we did not notice any significant linewidth broadening in the absorp- tion spectra, indicating that the higher growth rate does not deteriorate the interface quality between the layers.

3 Optical characterisation

The sample, approximately 5 mm long and 3 mm wide, was mirror-polished on the substrate side and on two opposite edges lapped into 45" wedges. For linear absorp- tion measurements, the sample was placed inside a beam condenser in a Fourier transform infra-red spectrometer; white light was passed through the sample in multiple bounce geometry and detected using a cooled InSb- detector. A large area polariser was used to select either P- or S-polarised light entering the sample. Bare sapphire and bulk-like GaN-on-sapphire samples were similarly prepared, and served as reference and background samples. Emission experiments were not performed.

The sample displayed a peak absorption wavelength of 1.67 ym (740 meV), and a full width at half maximum of the absorption of - 200 meV; therefore still with consider- able absorption at 1.55 ym (800meV). Fig. 2 shows the linear absorption spectrum as measured in a multi-pass configuration [SI, as well as a background spectrum. The inset shows a portion of the conduction band structure calculated using Schroedinger's and Poisson's equations iteratively.

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We extract an optical dipole matrix element of 0.4nm, and a LO phonon scattering time of 1 10 fs, respectively, for the IS transition of the main QW. These calculations, however, carry a significant uncertainty, as giant built-in piezo- and pyroelectric fields are expected to be present throughout the MQW structure [13]. The magnitude of these fields has not yet been measured (for such narrow QWs and large AIN-mole fraction barriers) and is also difficult to extract from our intersub-band studies. Although free electrons from doping are capable of partially screening the electric field, a significant distortion of the band profile is still quite likely. As a result, the electron wave fimctions of the lower and upper energy levels in the main absorbing QW are increasingly off-set with respect to each other. This leads to a decrease in the optical dipole matrix element and also an increase in the LO phonon scattering time.

The relaxation time between the two electron sub-bands was directly measured using the pump-and-probe photo- modulation technique. A synchronous optical parametric oscillator (Spectra-Physics) produced 120 fs laser pulses at 1.55 pm (the signal output) and 1.7 pm (the idler output), which were used as the pump and probe pulses, respec- tively; i.e. both the pump and the probe beam are exciting IS transitions. Both of the beams were collinearly focused with a 20x objective lens onto the sample, tilted by 45" with respect to the beams. The transmitted light was detected after only a single pass through the sample (in contrast to the multi-pass geometry used for linear absorp- tion measurements). We estimate the power density of the pump beam as - 100 W/cm2. The spectra of both pulses were within the IS absorption band (Fig. 3). Therefore, photo-excitation by the pump pulse resulted in the increase of the probe transmission ( T ) due to the partial bleaching of the IS absorption (i.e. positive ATIT). By varying the delay between the pump and probe pulses, we could monitor the IS relaxation dynamics.

Fig. 4 shows the photomodulation (ATIT) dynamics observed with the S- and P-polarisations of the beams (the pump and probe are always co-polarised). Since the electric field of the IS transitions is always polarised perpendicular to the sample plane, the strongest response is observed for the P-polarisation. From its initial decay, we infer the primary decay time constant of 370fs and

0 " " " " " " " " " " 1 .o 1.5 2.0 2.5 3.0

wavelength, pm

Fig. 3 Transmission spectrum (raw data) of GaNIAIGuN MQW struc- ture shown in Fig. 1 (-) and transmission spectrum (raw datu) ofpluin sapphire substrate (- - -) both measurements taken in multiple pass geometry, in P-polarisation [SI, and spectra matched in intensity: IS absorption is peaked at 1.67 pm inset: energy diagram of portion of conduction band of GaN/AIGaN MQW structure; open arrow indicates IS absorption process a.u. =arbitrary units

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time, ps

Fig. 4 AT/Tphotomodulution dynamics in GaNIAlGaN MQW structure for P-polarisation (-) and S-polarisation (- - -) . . . bi-exponential fit: 0.0002.exp(-~/0.37) + 0.00003.exp(-~/l .S), where 7 =time inset: sample orientation in set-up

attribute it to the IS relaxation time. The longer and weaker component in the relaxation dynamics in Fig. 3, with the time constant of 1.8 ps, can be due to the thermalisation of the electron distribution following the ultrafast IS relaxation.

4 Conclusions

We have investigated the growth of GaN epilayers using low-temperature A1N layers to reduce the threading dis- location density. For GaN/AIGaN MQWs, accurate layer thicknesses were achieved using growth rate information obtained in situ by pyrometric interferometry. Such a structure with GaN QWs and GaN/Alo h5Gao 35N short- period superlattice barriers was found to have a sub- picosecond IS electron scattering time (- 370 fs), with the sub-band spacing in the communications wavelength range. These results are very encouraging with respect to their potential use in ultrafast optical components.

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