Optical characterization of GaAs quantum wire microcrystals

Solid State Communications, Vol. 80, No. 3, pp. 235-238, 1991. Printed in Great Britain.

0038-1098/91 $3.00 + .00 Pergamon Press plc

OPTICAL CHARACTERIZATION OF GaAs QUANTUM WIRE MICROCRYSTALS G.P. Morgan,* K. Ogawa, K. Hiruma, H. Kakibayashi and T. Katsuyama Central Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185, Japan

(Received 4 July 1991 by G. Fasol) Optical properties of quantum-size GaAs wire crystals grown by organometallic vapor-phase epitaxy (OMVPE) are measured. The typical size of the wire-shaped microcrystals is 1-5 #m long and 10-200nm wide. Photoluminescence measurements at 4 K reveal spectral features dominated by free carrier to acceptor impurity recombination. A free exciton recombination line is also observed and is more intense relative to other features than that observed from a conventional OMVPE epitaxiaily grown layer. Small spectral shifts (0.5 meV) of the free exciton and the acceptor-bound exciton recombination lines are considered to be due to the effects of quantum confinement on the energy levels of the system.

IN RECENT years there has been considerable interest in the quantum effects of dimensional confinement on the optical properties of semiconducting materials [1]. These confinement effects provide a means of artificially tailoring the optical properties of materials at certain wavelengths. This could be useful in the production of optoelectronic devices. For example the modification of the carrier density of states enables the production of laser devices with lower threshold currents and higher gains [2]. The enhanced nonlinear optical properties also make certain lowdimensional structures suitable for other optoelectronic applications [3]. Two-dimensional quantum well devices such as quantum well lasers have been very successful [4]. Furthermore, one-dimensional quantum wires and zero-dimensional quantum dots are being studied extensively [5, 6]. However, fabrication techniques of such quantum wires and dots are not as advanced. Although quantum wires, for example, have been fabricated by focused ion beam technology [7], electron beam technology [8] and epitaxy using tilted substrates [9, 10], these techniques have not provided high-quality quantum wires due to surface damage, interface fluctuations, etc. On the other hand we recently developed a new method to grow one-dimensional GaAs wire-shaped microcrystals on a partly masked substrate by using low-pressure organo-metallic vapor-phase epitaxy (OMVPE) [11, 12]. This communication presents low temperature laser-induced photoluminescence * Present address: Department of Physics, University College, Galway, Ireland.

measurements of these microcrystals which suggest the possibility of quantum confinement effect in two dimensions. GaAs microcrystals were grown on (1 0 0) oriented GaAs substrates by the OMVPE method in a vertical SiO2 reactor at 460°C with an AsH3-to-trimethylgallium (TMG) molar flow ratio of 120. Selective growth occurred in the exposed windowed regions of an SiO2 mask which covered the substrate surface, resulting in the formation of GaAs wire-shaped microcrystals. The substrate was doped with chromium and oxygen and was semi-insulating. Figure 1(a) shows a scanning electron microscope (SEM) image of the surface of a selectively grown sample. Wire-shaped crystals grew in the windows in the SiO2 mask where the GaAs substrate is exposed. Figure l(b) is a SEM image of an individual microcrystal with a hexagonal-like cross-section which decreases in a stepped fashion from a diameter of 150 nm at the base to 20 nm at the peak. The crystals are typically 1/~m in length. The transmission electron diffraction patterns indicate that the microcrystals are single crystals with preferential axial growth along the [1 1 1] direction. The growth mechanism for such wireshaped microcrystals may be explained by the vaporliquid-solid (VLS) equilibrium phase growth model proposed by Wagner and Ellis for Si whisker growth [13]. In this model small Ga-rich seeds form on the SiO2 masked regions in the beginning of the growth. Then the seeds migrate on the GaAs substrate and form wire-shaped microcrystals [12]. Although Fig. l(a) shows a sparsely covered window region to emphasize the orientation of the crystals, the packing density of the microcrystals can be quite large. Therefore

235

236

Vol. 80, No. 3

GaAs Q U A N T U M W I R E M I C R O C R Y S T A L S

-~

1.520

1.510

,

]

ENERGY (eV) 1.500 i

1.490 i

4K

_d

Z U.I I-.Z

__i el"

I

815

825 830 835 WAVELENGTH (nm) Fig. 2. Photoluminescence at T = 4 K from window regions in a SiO2 mask, which contains a large density of wire-shaped GaAs mircrocrystals.

Fig. 1. (a) Scanning electron microscope (SEM) xmage of wire-shaped microcrystals, which are sparsely populated in a SiO2 window region of a GaAs substrate. The black region is a SiO2 masked region. Microcrystalline growth occurs in two preferred [1 1 1] directions. (b) High-resolution SEM image of a single GaAs microcrystal. A hexagonal cross section and stepped surface structure is apparent. microcrystals completely cover the exposed substrate surface. This means that the influence from the substrate can be eliminated in measuring photoluminescence spectra from the microcrystals themselves. The photoluminescence measurements were carried out at 4.2 K by immersing the samples in liquid helium. The 514.5nm emission from an argon-ion laser was used to excite the luminescence. The result-

820

ant photoluminescence was dispersed by a 1 m focal length Jobin-Yvon monochromator with a linear dispersion of 8 A m m ~. The transmitted light was detected with a cooled Hamamatsu R94302 GaAs photomultiplier tube. Lock-in signal processing was used to obtain an improved signal-to-noise ratio. Figure 2 shows the photoluminescence spectrum of the wire-shaped microcrystals at 4.2 K. The spectrum is dominated by a broad spectral feature at 830 nm due to free electron to neutral acceptor impurity recombination and a possibly unresolved neutral donor-to-acceptor feature at longer wavelengths. Excitonic features appear at shorter wavelengths of 817-820nm, which are presented in more detail in Fig. 3(a). The luminescence spectra from the bare semi-insulating GaAs substrate, Fig. 3(b), the SiO2 masked area with no window region, Fig. 3(c), and the conventional GaAs epitaxial-layer sample, Figure. 3(d), are shown. In Fig. 3(d), the GaAs epitaxial-layer photoluminescence is dominated by well-documented, sharp-line features. Those labelled "1" are due to a split free exciton decay [14] while features "2" and "3" are due to the neutral donor-bound exciton (D°,X) and neutral donor-bound hole (D°,h), respectively [15, 16]. The features labelled "4" are a result of the spin coupling interaction of a neutral acceptor-bound exciton. Because of the large central cell corrections for shallow acceptors, these features can be assigned unambiguously to carbon impurity acceptors [17]. The structure between features "1" and "2" has been reported as due to excited rotational states of the neutral donor-bound exciton complex [16]. The luminescence from the GaAs substrate [Fig. 3(b)] and the SiO2 masked areas [Fig. 3(c)], where

Vol. 80, No. 3

GaAs QUANTUM WIRE MICROCRYSTALS ENERGY(eV)

817

818 819 820 WAVELENGTH(nrn)

821

Fig. 3. Details of the T = 4 K photoluminescence in the 817-821 nm spectral region from (a) wire-shaped GaAs microcrystals, (b) a semi-insulating GaAs substrate, (c) a SiO2 masked area with no window region, and (d) a GaAs epitaxiaUy grown layer. very thin polycrystalline GaAs were grown on the SiO2 mask, is dominated by a broad neutral-carbon acceptor-bound exciton feature in this spectral region. Spin splitting is not observed due to the broadening effects of residual strain. No donor-related features are distinguishable. The luminescence from the wire-shaped microcrystals [Fig. 3(a)] is significantly different from that observed in Fig. 3(b)-(d). It consists of a free exciton peak (1.5154 eV) and a neutral carbon acceptor bound exciton peak (1.5125eV). Both peaks are shifted to a shorter wavelength. The neutral acceptor bound exciton emission has a distinguishable structure on the long wavelength side. Furthermore, it should be noted that the free excition emission is more intense relative to other features than that observed in Fig. 3(b)-(d). This corresponds to the relatively low impurity concentration, which indicates a high-quality crystalline form of the wire-shaped microcrystals. In the wire-shaped microcrystal system reported here, the free carriers and excitons are confined to the quantum wire surrounded by air, where the approximation adopted in simple models of an infinite confining potential should be quite realistic. The spectral shift of the various excitonic features is influenced

237

mainly by two processes. The first is the effects of quantum confinement on the free cartier energy levels. The electron and hole energies increase monotonically with increasing confinement. The second contribution comes from the binding energy of the electron and hole. This negative energy reduces the overall energy of the excited excitonic state. Variational techniques [18] show that for infinite confinement potentials the binding energy should increase monotonically with decreasing confinement dimensions. Numerical calculation shows that the free exciton feature should be shifted approximately 31 meV higher for an infinite potential quantum wire of diameter d = 20 nm. This shift decreases rapidly to 6 meV for d = 40 nm and is mostly due to the quadratic dependence of free carrier energy on d. When d = 200 nm, the shift is estimated to be less than 0.5meV. For a long tapered wire [Fig. l(b)], the resultant free-exciton spectral profile should reflect a convolution of features characteristic of each diameter. This convolution will also be weighted by the number of free excitons at each axial position in the wire. Figure 3(a) shows that the shift of the free exciton peak is less than 0.5 meV. The emission line is quite broad, however, and decreases slowly on the high energy side. This suggests that free exciton recombination occurs predominantly in the broad base region (d = 150 nm) of the microcrystals. If the excitons are created uniformly throughout the microcrystals this observation can then be understood in terms of an axial diffusion of excitons from the narrow free end of the crystals where the confinement energy is large to the base region along a decreasing confinement energy gradient. In a similar manner it can be argued that the non-equilibrium population of free electrons and holes will also diffuse to the base region. This is consistent with the observation that the (e, A°) recombination feature at 830 nm in Fig. 2 exhibits a negligible energy shift when compared to that of the substrate material. On the other hand the spectral shift of bound excitons is approximately 0.5meV, which is slightly greater than that of free excitons. The larger shift may result from the fact that spatial diffusion of the excitons to the base region is partly inhibited by the trapping effect of the neutral acceptors. Furthermore, the emission line is split in two with a distinct feature on the long-wavelength wing. We do not believe that this splitting is due to the effects of strain in the microcrystals [19, 20]. The main reason is that no similar splitting is observed for the acceptor-bound exciton emission from the polycrystalline GaAs grown on the SiO2 mask, where the large residual strain is expected because of the large

238

GaAs QUANTUM WIRE MICROCRYSTALS 2.

discrepancy between the thermal expansion coefficients of the SiO2 mask and GaAs. Furthermore, the residual strain cannot cause the large splitting of 0.6meV obtained in our experiment. Reynolds [20] reported that even if the strain is as large as 0.09kgmm -2, only a small splitting of less than 0.02 meV is obtained. To understand the origin of this splitting we note Bastard's work proposing the concept of an impurity band caused by the dimensional confinement effect of the electron and hole [21]. In his theory the luminescence spectra associated with electron-to-acceptor recombinations in the quantum well show two peaks because the binding energies of the acceptor levels depend on the positions in the quantum well due to the perturbation by the barrier potential. This confinement effect becomes much stronger in the one-dimensional quantum wire than in the two-dimensional quantum well. Therefore, we propose the concept of an impurity band in the quantum wire, which is essential in confined structures, as one possible origin of the two emission peaks. In summary, low-temperature laser-induced photoluminescence measurements have been carried out on GaAs wire-shaped microcrystals grown by the organo-metallic vapor-phase epitaxy (OMVPE) method. The microcrystals are typically 1/~m long with a base diameter of 150/~m decreasing to 20 nm at the peak. A distinct free-exciton recombination line has been observed. This emission intensity is more intense relative to other features than that observed from a conventional GaAs layer grown by OMVPE. Small spectral shifts (0.5meV) in the recombination lines of free excitons and acceptor-bound excitons are considered to be due to the effects of quantum confinement on the energy levels of the system.

18.

REFERENCES

19.

.

C. Weisbuch, in Applications of Muhiquantum Wells, Selective Doping, and Superlattice, Vol. 24, Semiconductors and Semimetals (Edited by R. Dingle), Chapter 1, Academic Press, New York (1987).

3. 4. 5.

6. 7. 8. 9. 10. 11.

12. 13. 14. 15. 16. 17.

20. 21.

Vol. 80, No. 3

Y. Arakawa & A. Yariv, IEEE J. Quantum Electron. QE-22, 1887 (1986). J.S. Weiner, D.S. Chemla, D.A.B. Miller, H.A. Haus, A.C. Gossard, W. Wiegmann & C.A. Burrus, Appl. Phys. Lett. 47, 664 (1985). W.T. Tsang, Appl. Phys. Lett. 39, 786 (1981). K.J. Vahala, J.A. Lebens, C.S. Tsai, T.F. Kuech, P.C. Sercel, M.E. Hoenk & H. Zarem, Proc. Int. Soc. for Optical Engineering (SPIE), Vol. 1216, pp. 120-129, Los Angeles (1990). D.B.T. Thoai, Y.Z. Hu & S.W. Koch, Phys. Rev. 1142 11261 (1990). Y. Hirayama, S. Tarucha, Y. Suzuki & H. Okamoto, Phys. Rev. B37, 2774 (1988). J. Cibert, P.M. Petroff, G.J. Dolan, S.J. Pearton, A.C. Gossard & J.H. English, Appl. Phys. Lett. 49, 1275 (1986). P.M. Petroff, A.C. Gossard & W. Wiegmann, Appl. Phys. Lett. 45, 620 (1984). T. Fukui & H. Saito, Appl. Phys. Lett. 50, 824 (1987). T. Katsuyama, G.P. Morgan, K. Ogawa, K. Hiruma & H. Kakibayashi, Technical Digest of XVII International Conference on Quantum Electronics, IQEC '90, QTUE6, Anaheim (1990). K. Hiruma, G.P. Morgan, T. Katsuyama, K. Ogawa, M. Kouguchi & H. Kakibayashi, Appl. Phys. Lett. 59, (4) (in press). R.S. Wagner & W.C. Ellis, Appl. Phys. Lett. 4, 89 (1965). E.S. Koteles, J. Lee, J.P. Salerno & M.O. Vassell, Phys. Rev. Lett. 55, 867 (1985). T.D. Harris, M.S. Skolnick, J.M. Parsey Jr. & R. Bhat, Appl. Phys. Lett. 52, 389 (1988). S.P. Watkins, G. Haacke & H. Burkhard, Appl. Phys. Lett. 52, 401 (1988). D.J. Ashen, P.J. Dean, D.T.J. Hurle, J.B. Mullin, A.M. White & P.D. Greene, J. Phys. Chem. Solids 36, 1041 (1975). J.W. Brown & H.N. Spector, Phys. Rev. B35, 3009 (1987). M. Schmidt, T.N. Morgan & W. Schairer, Phys. Rev. Bll, 5002 (1975). D.C. Reynolds, C.W. Litton, R.J. Almassy, G.L. McCoy & S.B. Nam, J. Appl. Phys. 51, 4842 (1980). G. Bastard, Phys. Rev. B24, 4714 (1981).