Optical phonon emission in GaAs/AlAs and GaAs/Al0.7Ga0.3As multiple quantum well structures

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Optical phonon emission in GaAs/AlAs and

GaAs/Al

_0.7

Ga

_0.3

As multiple quantum well structures

K.W. Sun

a,

*, C.M. Wang

a

, H.Y. Chang

a

, S.Y. Wang

b

, C.P. Lee

b

a

Department of Electronic Engineering, Feng Chia University, P.O. Box 25-239, Taichung 407, Taiwan, ROC

b

Department of Electronics Engineering and Institute of Electronics, National Chiao Tung University, Hsin Chu, Taiwan, ROC Received 25 February 2000; received in revised form 9 May 2000; accepted 9 May 2000

Abstract

We have performed Raman scattering measurements and hot electron–neutral acceptor (hot(e, A˚)) luminescence

experiments on Be-doped GaAs/AlAs and GaAs/Al0.7Ga0.3As multiple quantum well structures, with fixed well width of

50 A˚ and barrier thickness of 5, 25, 50,120 A˚, to determine the optical phonon energy emitted by the hot electrons excited in the quantum wells. It was shown that the relaxation of electrons in the GaAs layer is dominated by the AlAs-like optical phonon emission for samples with larger barriers, but by GaAs optical phonons for smaller barriers.

PACS: 78.55; 78.30; 78.60; 78.66

Keywords: Photoluminescence; Raman scattering; Optical phonon

1. Introduction

Semiconductor quantum well structures such as GaAs/AlGaAs systems not only have important device applications, but also provide a challenge to our understanding of the physics of hot carrier dynamics in semiconductors. Optical methods like reflection, transmission, and luminescence experi-ments are employed to characterize single and multiple quantum layers, to learn about recombi-nation mechanisms and the role of interfaces on these mechanisms. Raman scattering has been proven as a versatile and efficient tool for probing

long- and short-wavelength lattice dynamics of ternary alloys [1–5]. The electron–phonon interac-tions in semiconductor alloys have also been studied by using time-resolved Raman spectro-scopy [6–8]. It is well known that the AlxGa1ÿ xAs

barrier in the quantum wells is a two-mode alloy, in that two longitudinal–optical (LO) phonon modes of lattice vibration exist, even though the material is assumed to be a smooth alloy with average Al/Ga concentration on a single sublat-tice. More recently, Kash et al. [7] were able to evaluate the relative interaction strength of these two LO modes, with the electrons, using pico-second Raman spectroscopy.

In addition to the Raman scattering technique, it is well known that the radiative recombination of photoexcited carriers with the neutral acceptors *Corresponding author. Tel.: +886-4-4517250 e. 3877; fax:

+886-4-4510405.

E-mail address:[email protected] (K.W. Sun).

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can be used to the study the hot carrier relaxation processes. The relaxation of hot electrons through optical phonon emission in bulk GaAs [9–11] and heterostructures [12–14] has been extensively studied using above techniques. Sapega [15] has demonstrated that, for GaAs/AlAs quantum wells with large barrier widths, the energy relaxation mechanism for hot electrons is dominated by the AlAs phonons, For smaller barriers, emission via GaAs phonon is more important. By using conventional hot electron luminescence techni-ques, Ozturk et al. [16] have demonstrated that in GaAs/AlAs the dominant electron relaxation mechanism is via the interaction with the AlAs interface mode for a device having a well width of 80 A˚. But for a similar GaAs/Al0.24Ga0.76As

structure, the GaAs phonons provide the energy relaxation. Recently, Mirlin et al. [17] have studied electron relaxation in GaAs/AlAs quantum wells with fixed barrier width of 10 nm and well width varying from 4 to 13 nm. It was shown that, for larger wells, the electron relaxation is dominated by GaAs LO phonons. But in the smallest well width sample, it is dominated by AlAs optical phonons.

2. Experimental techniques

In this work, we first use Raman spectroscopy to determine the optical phonon energies in GaAs/ Al0.7Ga0.3As and GaAs/AlAs quantum well

sam-ples of different barrier widths. With the measure-ments of the energy separation of peaks in the hot electron-neutral acceptor luminescence spectra and the LO phonon energies retrieved via Raman experiments, we then analyze the type of optical phonon emitted by hot electrons during relaxation processes in the quantum wells.

The samples investigated were grown by mole-cular-beam epitaxy on (1 0 0)-oriented undoped semi-insulating GaAs substrate. The MQW sam-ples studied here were 50 A˚ GaAs wells, with Al0.7Ga0.3As or AlAs barrier of 5, 25, 50, 120 A˚ in

thickness, respectively. The central regions of 1 nm of the GaAs layer were doped with Be to 1018cmÿ 3. Two exciting lines were used for the Raman experiments: an Ar+ laser operated at

514.5 nm and a dye (DCM) laser operated on 655 nm. About 150 mW of the laser power was directed on the samples which were kept in a closed-cycle refrigerator at 15 K. Raman spectra were obtained in back scattering geometry and the scattered light was collected by a camera lens and passed through a notch filter before entering the spectrometer. The spectra were recorded with a combination of a SPEX 0.6 m triplemate spectro-meter equipped with a liquid nitrogen cooled CCD detector. For the excitation of hot electron – neutral acceptor luminescence, a dye laser (DCM) pumped by an Ar+ laser was used. The dye laser was operated at appropriate photon energies to excite all samples in order to give same amount of excess kinetic energies to electrons. The hot electron luminescence was analyzed with the same spectrometer and detector in the Raman experiments.

3. Results and discussion

The Stokes Raman spectrum measured in back-scattering geometry zðx0_x0_{Þz (where z and z are}

the directions of propagation of the incident and scattered laser beams, respectively, normal to the layers, and x0 _{is the corresponding polarization}

vector along (1 1 0) in the plane of the layers) de-tects the LO phonon modes of the samples. Fig. 1(a) and (b) show the Raman spectra for GaAs/Al0.7Ga0.3As and GaAs/AlAs quantum well

samples with different barrier widths excited with Ar+ laser. At the bottom of each figure we have placed the Raman spectrum of the bulk GaAs sample for comparison. In Fig. 1(a), the GaAs LO phonon mode is at 36.7 meV and, for the Al0.7Ga0.3As layers, the optical phonons display

a two-mode behavior: the GaAs-like (whose energy is below the GaAs LO phonon energy) and AlAs-like modes (whose energy is below the AlAs LO phonon energy). However, in Fig. 1(b), due to different compositions of the barriers only the AlAs-like and the GaAs phonon mode were observed. Our detection system is not capable of resolving the splitting of the GaAs LO phonon into confined modes and there is also no evidence of scattering from interface phonons [8]. Please

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note the broadening and asymmetry nature of the peaks which is due to the alloy potential fluctuation [18].

In Fig. 2(a) and (b), we have plotted the AlAs-like and/or GaAs-AlAs-like phonon frequencies as a function of barrier width at two different excita-tion wavelengths. In Fig. 2(a), we found that both the GaAs-like and AlAs-like phonon frequencies kept relatively constant throughout the whole range of the barrier width. However, for samples with AlAs barriers, the AlAs-like phonon frequen-cies approach those of the phonons in AlAs with increasing barrier widths as shown in Fig. 2(b). No

dependence of the phonon frequencies was found with the two different excitation wavelengths. We have also measured the anti-Stokes Raman spec-tra, but found no evidence related to the phonon absorption by photons. We attribute this to the vanishingly small thermal occupation of the LO phonon modes at very low temperature.

In Figs. 3 (a) and (b), we have shown the hot electron-neutral acceptor luminescence spectra from GaAs/AlAs and GaAs/Al0.7Ga0.3As

quan-tum well samples. The principles of this technique were given in Ref. [19]. The schematic of this technique is also shown in the inset of Fig. 3(a). The peak labeled ‘‘unrelaxed peak’’ in each spectrum corresponds to recombination of elec-trons, from the state at which they were created, with a neutral acceptor. The peak labeled ‘‘1’’ represents electrons recombining with neutral acceptors after emitting one LO phonon. The width of the peaks is determined by the electron energy distribution at the point of generation, Fig. 1. Raman spectra of (a) GaAs/Al0.7Ga0.3As multiple

quantum wells and bulk GaAs samples and (b) GaAs/AlAs multiple quantum wells and bulk GaAs samples at 15 K in the back scattering geometry for incident wavelength of 514.5 nm. The peak labeled GaAs mode in (a) is the LO phonon arising from the GaAs wells. The other two peaks labeled GaAs-like and AlAs-like modes are related to the Al0.7Ga0.3As barrier

layers. Only the AlAs-like LO phonon mode was observed from the barriers in (b).

Fig. 2. The AlAs-like LO phonon frequencies (square) and GaAs-like LO phonon frequencies (circle) from (a) GaAs/ Al0.7Ga0.3As multiple quantum wells and (b) GaAs/AlAs

multiple quantum wells were plotted as a function of barrier width at incident wavelengths of 514.5 and 655 nm.

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which is related to heavy hole subband warping, as well as the energy distribution of acceptors, the final state of recombination for the hot lumines-cence process.

The photoexcited carrier densities are deter-mined from the laser spot size on the sample and

the absorption coefficient of the QW samples at the excitation wavelength. The power density of the laser used for the excitation was about 10 W cmÿ 2, which resulted in a 2D carrier density of about 109cmÿ 2. Our photoexcited carrier densities are low enough so that the main mechanism of energy relaxation in the sample studied is the emission of optical phonons and the phonon–plasmon coupling can be ignored. In order to demonstrate the change of the lumines-cence spectra with different barrier widths and compositions, we have centered the first unrelaxed peaks in the spectra for all four samples. The separation of the ‘‘unrelaxed peak’’ and ‘‘1’’ peaks in the spectra should allow one to determine the energy of the phonons emitted by hot electrons during the relaxation processes. In order to determine the energy separation more accurately, we first subtract the background (which was originated from the band-to-band recombination) from the spectra and the energy spectra of the two remaining peaks were then fitted by Gaussian distributions. The energy difference between the two peaks is plotted for all the samples as a function of barrier width as shown in Fig. 4. For samples with the largest barrier, the phonon energies emitted by the electrons in the GaAs/ Al0.7Ga0.3As and the GaAs/AlAs quantum wells

approach 360 and 380 cmÿ 1, respectively. In the cases with smallest barriers, the emitted phonon Fig. 3. Hot electron luminescence spectra of (a) GaAs/AlAs

multiple quantum wells and (b) GaAs/Al0.7Ga0.3As multiple

quantum wells plotted as a function of the electron energy above the ground state of the quantum wells. The vertical line labeled ‘‘unrelaxed’’ is the energy peak corresponding to recombination at the energy of creation. The peak labeled ‘‘1’’ represents the electron distribution after the emission of one LO phonon. The inset shows schematically the principles of the hot electron–neutral acceptor luminescence technique.

Fig. 4. Measured energy separations between the ‘‘unrelaxed’’ and ‘‘1’’ phonon peaks in the hot electron luminescence spectra from GaAs/Al0.7Ga0.3As (dot) and GaAs/AlAs (square)

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energies measured from both samples approach 300 cmÿ 1, which is still higher than the GaAs LO phonon energy ÿ 293 cmÿ 1_.

In according to Ref. [15], the emission strength of a particular phonon mode is proportional to the square of the overlap integral of the phonon scalar potential with the initial and final electron states in the GaAs layer. The different relaxation paths were weighted byðjGa=jA1Þ2, where jGa is the sum for

the scalar potentials for all the calculated GaAs modes and j_A1 the sum of the AlAs modes at particular barrier width. In their works, for samples with smaller barrier widths, relaxation through the emission of GaAs modes is more important. But, for the 8 nm barrier widths, the energy relaxation was dominated by the AlAs phonons.

In our experiments, the monotonic increase of the energy separation between the peaks (the phonon energies emitted by hot electrons) in the hot electron luminescence spectra suggests that the coupling strength between hot electrons and AlAs-like phonons is becoming stronger as the barrier width is increased. Therefore, we can esti-mate the emission strength of AlAs-like LO phonons relative to the GaAs LO phonons by taking into account the optical phonon energies measured in the Raman experiments and the energy separations in the hot electron lumines-cence spectra. In Fig. 5, we have plotted the fractional emission strength of the AlAs-like

optical phonon relative to the GaAs LO phonon as a function of barrier width for both samples by assuming that the emitted phonon energy was partitioned by AlAs-like and GaAs LO phonons. In the case for the smallest barrier width, the energy separation of the peaks is still about 13 cmÿ 1 larger than the energy of the GaAs LO phonons. This indicates that although interaction with the GaAs LO phonon is strong, there is still a significant contribution from the AlAs-like LO phonon. However, for the larger barrier, the spectra are dominated by AlAs-like LO phonons and the energy separations are very close to the AlAs LO phonon mode. We have also found that the fraction of the AlAs-like mode increases more rapidly in samples with AlAs barriers as the barrier width is increased in comparing to the GaAs/Al0.7Ga0.3As quantum wells.

Investigations of the GaAs/AlAs multiple-quantum wells via high-temperature mobility measurements [16] have also demonstrated the substantial influence of the AlAs-like LO phonon modes on the hot electron relaxation processes. On the contrary, the GaAs phonons provide the energy relaxation in a similar GaAs/Al0.24Ga0.76As

structure. The predominance of the AlAs-like phonon modes is attributed to the stronger scattering strength and to their shorter lifetime compared to the GaAs modes. Our results have indicated that, for quantum wells with AlAs barriers, the hot electrons relax mostly via the AlAs-like optical phonon emission with barrier width larger than 25 A˚. However, in simi-lar samples with Al0.7Ga0.3As barriers, the

AlAs-like mode emission dominates the relaxation processes only when the barrier width is larger than 100 A˚.

4. Conclusion

In conclusion, we have observed phonons in the present Raman scattering and hot electron-neutral acceptor luminescence investigation of the GaAs/ Al0.7Ga0.3As and GaAs/AlAs multiple-quantum

wells. In the Raman scattering experiments, we have observed two-mode behavior from Al0.7Ga0.3As barriers, but only one LO phonon Fig. 5. The estimate emission strength of the AlAs-like LO

phonon mode relative to the GaAs LO phonon as a function of the barrier width from both samples.

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mode is observed from AlAs barriers. The AlAs-and GaAs-like phonon frequencies were measured as a function of barrier width. We have also demonstrated that, for smaller barrier, the emis-sion of the GaAs optical phonon mode is stronger. But for the largest barrier investigated, the energy relaxation of hot electrons is dominated by the AlAs-like phonon.

Acknowledgements

This work was supported by the National Science Council of Republic of China under contract Grant No. NSC87-2112-M-035-004 and NSC88-2112-M-035-001.

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