ZnS strained-layer superlattices

PHYSICA

Physica B 191 (1993) 102-118 North-Holland SDI: 0921-4526(93)E0091-T

Nonlinear optical properties and blue-light-emitting diode of ZnSe / ZnS and ZnTe / ZnS strained-layer superlattices Toshiya Yokogawa 1 Opto-Electronics Research Laboratory, Semiconductor Research Center, Matsushita Electric Industrial Co. Ltd., Osaka, Japan

This paper reviews recent work on the nonlinear optics of ZnSe/ZnS and ZnTe/ZnS strained-layer superlattices (SLSs), with emphasis on their use in novel devices. The transmission spectra of ZnSe/ZnS SLS grown on CaF 2 reveal the importance of excitonic processes, showing large energy shifts due to quantum confinement and strain. Refractive index modification and layer disordering of such superlattices results from implantation of N ÷ and Li ÷ ions. A transverse electric field spatially separates charge carriers. This leads to changes in the photoluminescence and optical transmission of ZnSe/ZnS SLS. Optical switching devices and optical frequency doublers are demonstrated. By analysing the excitation power dependence of PL from ZnTe/ZnS doping superlattices, it is confirmed that the dominant emission process involves recombination across the indirect gap in real space between conduction band electrons and impurity band holes. Blue-light-emitting diodes at room temperature were fabricated using p-ZnTe/ZnS doping SLS. It is clear that the optical properties of the ZnSe/ZnS and ZnTe/ZnS SLS are very useful for novel opto-electronic devices in the visible spectral region.

I. Introduction C o m p a c t short-wavelength c o h e r e n t light sources are required for application in optical-information processing or display devices. Recently, laser diodes (LDs) have a p p e a r e d at the wavelength of 490 n m using C d Z n S e / Z n S e q u a n t u m well structures [1,2]. I I - V I c o m p o u n d semiconductors like Z n S e , ZnS and CdS are promising materials for application on short-wavelength optical devices with nonlinear optical properties, b e c a u s e they have wide b a n d gaps and show strong r e s o n a n c e of excitons, even at r o o m t e m p e r a t u r e , due to the large binding energy. F o r instance, the nonlinear optical properties in I I - V I s are useful for p h o t o n i c switching or logic

Correspondence to: T. Yokogawa, Opto-Electronics Research Laboratory, Semiconductor Research Center, Matsushita Electric Industrial Co. Ltd., 3-1-1 YagumoNakamichi, Moriguchi, Osaka 570, Japan. ~Present address: Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106, USA.

devices in the visible spectral region. F r e q u e n c y doubling using second h a r m o n i c g e n e r a t i o n ( S H G ) in I I - V I nonlinear materials is also an attractive m e t h o d for obtaining s h o r t - w a v e l e n g t h c o h e r e n t light. It is k n o w n that optical nonlinearities of semiconductors are greatly e n h a n c e d in superlattice structures over those of bulk crystals because of the r e m a r k a b l e persistence of the exciton resonances at r o o m t e m p e r a t u r e [3-5]. Since superlattice structurers are also e x p e c t e d to have a large absorption change due to the q u a n t u m confined Stark effect, m u c h attention has b e e n paid to the d e v e l o p m e n t of high speed optical switches [6-8]. In this paper, nonlinear optical properties and application to short-wavelength optical devices of Z n S e / Z n S and Z n T e / Z n S strained-layer superlattices (SLSs) are described. First, the growth of Z n S e / Z n S strained-layer superlattices on C a F 2 substrates is reported. T h e optical properties of Z n S e / Z n S SLSs on C a F 2 is analysed. Second, layer disordering of Z n S e / Z n S SLSs by N ÷ or Li ÷ ion implantation is r e p o r t e d ,

0921-4526/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

T. Yokogawa / On ZnSe/ZnS and ZnTe/ZnS strained-layer superlattices

and a large refractive index change induced by interdiffusion in SLSs is demonstrated. Third, the effect of an electric field on photoluminescent properties of ZnSe/ZnS SLSs is reported, and an application of ZnSe/ZnS SLSs to optical modulators is described. As evidence of the optical nonlinearity of ZnSe/ZnS SLSs, the observation of SHG in the ZnSe/ZnS SLS using either YLF laser or AIGaAs/GaAs semiconductor laser is reported. Fourth, the growth of ZnTe/ ZnSTe doping and type II superlattices (SLs) on InP substrates is reported. The properties of ZnSTe ternary alloy is also described. Finally, p - n junction blue-light-emitting diodes (LEDs) using p-ZnTe/ZnS doping SLs are proposed and successfully demonstrated.

2. Optical properties of ZnSe/ZnS strainedlayer superlattices grown on CaF 2 substrates

II-VI semiconductors such as ZnSe and ZnS, having wide band gaps and large nonlinear optical coefficients, are promising materials for use in SHG devices with application as shortwavelength coherent light sources. One promising approach is a Cerenkov-type SHG device for obtaining high efficiency frequency doubling because the phase matching condition between the fundamental guided code and the secondharmonic (SH) radiation mode can be automatically satisfied with the correct waveguide parameter [9]. In this approach, the SH radiation should pass through the substrate material on which the waveguide is formed. Therefore, in order to realize the blue light generation by the Cerenkov doubling, CaF 2 is a suitable substrate to use for the growth of II-VIs, as it is transparent to blue light. The lattice constants of group IIa fluorides such as CaF2, SrF 2 and BaF 2 can be matched to that of II-VI semiconductor by using mixed crystals of these fluorides. The possibility of Cerenkov-type phasematching by using the structure of ZnSe/ZnS SLS grown on CaF 2 is confirmed. Calculation of the SH power in the SLS/CaF 2 waveguide structure is carried out, following methods indicated in the literature [9]. Figure 1 shows the relationship

103

© O f.D

CaF2 sub.

2

m

o < o

0

I

50

0

100

150

FILM THICKNESS (nm) Fig. 1. Relative output power of second-harmonic generation vs film thickness.

between SHG power and film thickness. It is assumed that the wavelength of the fundamental mode is 0.8 ~m and the waveguide length is 1 mm. The refractive indices used in the calculation are 2.3 for 0.8 Ixm and 2.5 for 0.4 p,m in SLS, and 1.45 for 0.8 Ixm and 1.5 for 0.4 Ixm in CaF 2. This calculation shows that it is possible to obtain the Cerenkov-type phase-matching condi-

Ee-hh~" / ZnS (5 nm) k ~ 7 periods

1

~e-m\ ~. z" ZnSe (5 nm)

Z 0

/ ZnS (3 nm) ~

periods

1 Z ~ Z n S e - -

500

(8 nm)

/ ZnS (3 nm) 40 periods

I

I

I

450

400

350

300

WAVELENGTH (am) Fig. 2. Transmission spectra obtained at 77 K in three ZnSe/ ZnS strained-layer superlattices.

104

T. Yokogawa / On ZnSe/ZnS and ZnTe/ZnS strained-layer superlattices

tion in the SLS/CaF z structure although a particular film thickness exists for which the SH power is maximally generated or not at all. Z n S e / Z n S SLSs were grown on (1 0 0) CaF 2 substrates directly using metalorganic vapor phase epitaxy (MOVPE). Dimethylzinc (DMZ), HzSe and HzS were the source gases for Zn, Se and S, respectively. Before the growth, the substrate was chemically etched in HzSO 4 solvent. The substrate temperature was 400°C and the reactor pressure during growth was 75 Torr. For characterizing the optical properties of SLSs, transmission measurements and photoluminescence (PL) measurements were carried out at 77K. In the transmission measurements, the optical beam passed normal to the layers. In the photoluminescence measurements, a 10 mW H e Cd laser was the excitation source. The Z n S e / Z n S SLSs comprise 57 periods of 3 nm ZnSe and 5 nm ZnS, 57 periods of 5 nm ZnSe and 3 n m ZnS, and 40 periods of 8 n m ZnSe and 3 nm ZnS. Single-crystalline SLSs with mirror surfaces were obtained at a growth temperature of 400°C. Figure 2 shows typical transmission spectra of ZnSe/ZnS SLSs in the bandedge region of ZnSe at 77 K. Doublet absorption peaks due to excitons are observed, which may be related to the An = 0 , heavy hole (n = 1), electron (n = 1) transition (Ee_hh) and the light hole (n = 1), electron (n = 1) transition (Ee_lh). As the ZnSe layer thickness decreases, these peaks shifts toward higher energies. For the case of a free-standing ZnSe/ZnS SLS, the equilibrium in-plane lattice constant of the superlattice is given [10] by a znse G z n s ¢ L z n s e + a z n s G z , s L z , s

all = Gi = 2(ciH

GznseLzns¢ + GznsLzns i + 2C,2)(1 - C , i2 / C n )i

'

(1) (2)

where i denotes the material, ZnSe or ZnS, a i is the lattice constant of the bulk crystal i, and Cin i and C12 are the elastic constants of i. The lattice constant of CaF 2 (0.58 nm) lies between those of ZnSe and ZnS. Therefore, for a particular ratio of the thickness of ZnSe and ZnS, the equilibrium in-plane lattice constant all of SLS can

10

ZnSe (8 nm) / ZnS (3 nm) @/c'/ ZnSe (5 am) / ZnS (3 nm)

nm)

0

I

0

5

10

15

dzns (nm) Fig. 3. Relationshipbetween ZnSe and ZnS layer thickness when the equilibriumin-plane lattice constant of the superlattice matches that of CaFv

match that of CaF2, according to eqs. (1) and (2). Figure 3 shows the relationship between ZnSe and ZnS layer thickness in the lattice matching condition, calculated using eqs. (1) and (2). The solid circles in fig. 3 correspond to the samples characterized by transmission measurements. This figure shows that the in-plane lattice constant of the 3 nm ZnSe/5 nm ZnS SLS matches that of CaF 2. Transmission measurements show that the excitonic structure in the 3 nm ZnSe/5 nm ZnS SLS has the highest resolution in these samples, which indicates the good quality of the two-dimensional structure. These resuits show that in order to obtain good crystalline quality of SLS on CaF2, lattice matching between in-plane constant of SLS and CaF 2 is preferred. The individual layers in the SLS match up with lattice constants in the planes parallel to the interface by compression or expansion. Due to the Poisson effect, these layers are expanded or compressed in the direction perpendicular to the interface. In the case of biaxial in-plane compression, the fourfold ( J = 3 / 2 ) valence band splits because of lowering in the symmetry from T d to D2d. In addition, the hydrostatic component of the stress shifts the center of gravity of fourfold (J = 3/2) and twofold (J = 1/2) multiplets relative to the bottom of the lowest conduction band. When the perturbation expansion is pursued to the first two terms in the deformation

T. Yokogawa / On ZnSe/ZnS and ZnTe/ZnS strained-layer superlattices

theory, the calculated energy difference between heavy hole (EtcH) and light hole (ELH) band at k---0 is given [11,12] by EH H -

EL H = 2bSexx - 2 b z S 2exxA 2

(3)

3.1

105

5Ev = 0.84 eV mhh = 1.44m0

>

77 K 3.0

where zi is the spin-orbit coupling. The notation S---($11- S , 2 ) / ( S , , + S,2) is used. b is the deformation potential (1.2 eV for ZnSe). exx is the strain in the layer. Assuming a free-standing superlattice, the energy difference between Ee_hh and E¢_lh is calculated by eq. (3). Figure 4 shows the comparison between the calculated curve and experimental data. The solid and dashed curves are calculated using only the first term of eq. (3) and both terms of eq. (3), respectively. As shown in this figure, the energy separation between E~_hh and E~_m increases with decreasing ZnSe layer thickness, which corresponds to an increase in the strain of the ZnSe well layer, according to eqs. (1) and (2). The experimental data are in fairly good agreement with the calculations. These results confirm that the two peaks are related to the An = 0, heavy hole (n = 1), electron (n = 1) transition and the light hole (n = 1), electron (n = 1) transition. The quantum size effect of SLSs are illustrated 0.3 ZnSe (3 nm) / ZnS (5 n m ) . ~:>

0.2

ZnSe (5 nm) -

/ ZnS (3 nm)

~

0.1

~

t "

~"" ~ P

0

I

0

ZnSe (8 rim) / ZnS (3 rim) I

I

I

0.01 0.02 0.03 0.04 0.05 e II ( = (aZnSe "a I1 ) / a z n s e ~

Fig. 4. Energy difference between heavy hole (Ec_b~) and light hole (Eo_,h) peak energies vs strain in ZnSe well layer.

Z <

2.9

2.8

I 0 5 10 ZnSe LAYER THICKNESS (nm)

Fig. 5. Dependence of heavy hole peak (E¢ hh) energy on ZnSe well layer thickness. Triangles and circles were obtained from transmission and photoluminescence measurements at 77 K, respectively.

in fig. 5, which shows the relationship between peak energy and ZnSe well layer thickness. Triangles and circles in fig. 5 are obtained from the peaks in transmission and emission spectra at 77 K, respectively. As the ZnSe thickness decreases, the emission peak shifts towards higher energy. Assuming that the discontinuity of the valence band AEv is 0.84 eV (ref. [13]) and the heavy-hole mass mhh is 1.44m 0, the transition energy Ee_hh was calculated. A free-standing superlattice and a constant exciton binding energy of 20 meV were assumed. The solid curve shows the calculated result using a Kronig-Penney model, which is in fairly good agreement with the experimental data. From the relationship between ZnSe well layer thickness and peak energy, the quantum size effect in the quantum well is also confirmed. Small Stokes shifts were observed between absorption and emission peaks. Part of the Stokes shift may be due to interface fluctuations. Growth of ZnSe/ZnS strained-layer superlattices on C a F 2 substrates has been demonstrated for the first time. The growth of high quality

106

T. Yokogawa / On ZnSe/ZnS and ZnTe/ZnS strained-layer superlattices

Z n S e / Z n S SLSs on CaF 2 is very useful for Cerenkov-type SHG devices. 10~

3. Disorder of ZnSe/ZnS strained-layer superlattices Impurity-induced disordering of semiconductor superlattices is a useful technique for patterning the refractive index and bandgap distribution in the plane of the SL layer [14-16]. This capability provides the foundation for new types of semiconductor devices as well as optoelectronic integrated circuits. Recently, the possibility to modify the structure of the ZnSe/ZnS SLS using Si ion implantation at a dose of 1 × 10 a6 ions/cm 2 [41] was demonstrated for the first time. However, such a high dose implantation of Si has the disadvantage of creating defects which are not eliminated by subsequent thermal annealing. N ÷ and Li* are preferred as ion species because the radiation damage can be removed easily, the diffusion constants are high, and these impurities do not generate deep levels such as self-activated centers. Layer disordering of ZnSe/ZnS SLSs was produced by N* or Li + ion implantation. ZnSe/ ZnS SLSs have been grown by low-pressure M O V P E using D M Z , dimethylselenium (DMSe) and H2S. Substrates are Cr-doped (1 0 0) GaAs. The growth temperature and pressure are 550°C and 75 Torr, respectively. One structure used for the implantation experiments consists of a 40 period superlattice with alternating layers of 3 nm ZnSe and 7 nm ZnS. Ion implantations were performed at room temperature with an energy of 100 keV at an angle of 7 °. Samples were implanted using either a 5 x 1014 ions/cm 2 dose of N ÷ or a 1 × 1015 ions/cm 2 dose of Li ÷. The calculated projected ranges Rp of N and Li are 0.18 and 0.35 p~m, respectively, by the Lindhard-Scharff-Schiott (LSS) theory. Post-annealing was carried out in a Se and S ambient at 400°C. The Se and S depth profiles were measured by secondary-ion mass spectrometry (SIMS), using a Cs ÷ primary-ion beam at 1.5 keV. PL measurements were carried out using a 325 nm He-Cd laser (10 mW) for excitation.

10 3 -

¢-

(a)

10 2

101 >-

~-

10 3 -

O3 Z

(b)

f

lo 2 Z

10 0 I

0

I

20

t

J

I

I

I

I

[

40 60 80 DEPTH {nm}

Fig. 6. Depth profiles of S by SIMS analyses (a) just after growth and (b) following N ÷ ion implantation and subsequent thermal annealingat 400°C for 3 h. The superlattice comprises 40 periods of 3 nm ZnSe and 7 nm ZnS. ZnSe/ZnS interdiffusion was investigated using SIMS. Figure 6 shows the S depth profiles for the SLS (a) after growth, and (b) after N + implantation and subsequent anneal at 400°C for 3 h. In fig. 6(a), the superlattice structure is clearly observed, but in fig. 6(b), the periodic profile has disappeared. These results indicate that the superlattice structure is disordered by the thermal anneal after the implantation. The refractive indices of the SLSs are estimated by reflectance measurements and computer simulation. These indices are calculated by using the reflectance measured from superlattice layer surface at normal incidence. Since the superlattice layer is a Fabry-Perot etalon sandwiched by air and the GaAs substrate, an interference ripple is observed in the spectrum of reflectance. In order to estimate the refractive indices, the reflectance spectrum of the a i r / I I V I / G a A s system was simulated, assuming that the single-effective-oscillator approximation is valid for the II-VI material at photon energies

T. Yokogawa / On ZnSe/ZnS and ZnTe/ZnS strained-layer superlattices

sufficiently below the direct band edge [17]. The values for the complex refractive index of GaAs are taken from ref. [18]. Two energies E a and E0 (of eq. (10) in ref. [17]) are used as fitting parameters. In the best fit of the single-effectiveoscillator formula, the results of the simulation are in extremely good agreement with the measured reflection spectra. Figure 7(a) and (b) show the dispersions of the refractive index in N ÷- and Li+-implanted SLSs, respectively. A significant decrease in the refractive index is observed in both SLSs which are N ÷ and Li ÷ implanted and annealed at 400°C for 3 h. This behavior is very useful for the fabrication of optical devices in ZnSe/ZnS SLSs. PL measurements at 77 K also show the layer disordering of ZnSe/ZnS superlattices. Before

3.4

ZnSe/ZnS SUPERLATTICE R.T.

(o)

3.2 x

3.0

UNIMPLANTED

z 2.8 w 2.6 2.4 h W '~ 22 20450

"N'r IMPLANTED ~ N + IMPLANTED 5xl01'~cm"z SxlO~cm.2 ~O0"C, Stain ANNEAL 400"(:, 3h ANNEAL I I I J I I I I 500 5S0 600 650 700 750 800 8S0 WAVELENGTH (nm)

3.2 t

the ion implantation, the excitonic emission line, which is related to the transition between quantum energy levels in ZnSe wells, was observed at 2.925eV. Just after the ion implantation, no emission was observed because of crystalline damage due to the implantation. However, emission was observed after ion implantation and annealing, which indicates the recovery of crystalline quality. After annealing at 400°C for 1.5 h, a strong excitonic emission near 3.1 eV was observed in implanted SLSs. Figure 8 shows the relationship between the PL peak energy and the annealing time in both N+-implanted and Li ÷implanted superlattices. The post-annealing is done at 400°C for 1.5 h and for 3 h. A slight red shift of the PL peak was observed in the early stage of annealing. The difference of bandgap energy between the strained ZnSe well layer and the bulk ZnSe was calculated by the deformation theory, [19] assuming that the SLS is free-standing. A value of 59 meV (AEcalc in fig. 8) was found, which agrees well with the red shift of the PL peak. Therefore, it is confirmed that this shift is related to the relaxation of misfit strain by partial interdiffusion. For longer annealing times, the PL peak for the implanted SLS shifts

3.2 Li

900 ~'3.1

/

3.4 I- ZnSe/ZnS SUPERLATTICE

107

(b)

R.T.

~

3.0

z

2.9

n~

N

/ -

~ 2.8 ¢'-'N-- Eg bulkZnSe

! 2.8 ~____~NIMPLANTED

n

27 "

I

P:!.uPL TE° Ix10~cm "z

22t

2oi

450

,.

14;~)O~:ns:in ANNEAL

~

500

=

550

i

=

L.P IMPLANTED ~s -2

2.6

~ ( ~ ! i m h ANNEAL

=---~-

600 650 700 750 WAVELENGTH (nm)

,

800

j

850

900

Fig. 7. (a) Refractive indices for the as-grown superlattice and for superlattices annealed following N + implantation as a function of wavelength. (b) Refractive indices for the asgrown superlattice and for superlattices annealed following Li ÷ implantation as a function of wavelength.

I

0

I

I

I

1.5 3.0 4.5 ANNEALING TIME (hours)

6.0

Fig. 8. Photoluminescene peak energy vs annealing time for superlattices which were N+-implanted (dashed line) or Li +implanted (solid line). AEca~cshows the difference of bandgap energy between the strained ZnSe well layer and bulk ZnSe calculated by deformation theory. The plot at zero annealing time corresponds to the emission peak for the as-grown superlattice.

T. Yokogawa / On ZnSe/ZnS and ZnTe/ZnS strained-layer superlattices

108

towards the higher energy side, which clearly indicates layer disordering of the SLS. Temperature dependences of PL show a reduction of quasi-two-dimensional exciton binding energy with disordering. Figure 9 shows the PL intensity as a function of reciprocal temperature. PL quenching properties, which are related to the dissociation of exciton and strong electronphonon interaction [20], are observed in both samples. It is already reported that the PL quenching properties in SLSs strongly depend on the exciton binding energy which increases with decreasing ZnSe well layer thickness. In asgrown ZnSe/ZnS SLSs, a strong excitonic emission and a large activation energy of quenching are obtained due to the large oscillator strength and large binding energy of quasi-two-dimensional excitons in a quantum well. However, the activation energy for disordered SLS is estimated to be 16 meV, much smaller than that for the as-grown SLS. These results indicate that the confinement of excitons in quantum well is weakened by the interdiffusion in the heterointerface of SLS, which results in a reduction of the exciton binding energy. Consequently, the large change of oscillator strength as well as the energy gap induces the large refractive index change. Interdiffusion in ZnSe/ZnS SLSs induced by low-damage N ÷ or Li ÷ ion implantation and

100 t,5me'v" t'-

:>..

~

,

o

(b)

10

c

I--

z Ltl

I--

z

I

I

I

I

1

I

I

0 1 2 3 4 5 6 7 RECIPROCAL TEMPERATURE (xlO "2K1) Fig. 9. Photoluminescence intensity for (a) the as-grown superlattice and (b) the disordered superlattice as a function of the reciprocal measuring temperature.

low-temperature annealing is confirmed. This low-temperature planar process will be very useful for the fabrication of II-VI semiconductor optoelectronic devices.

4. Effect of electric field on ZnSe/ZnS strainedlayer superlattices

Much attention has been paid to superlattice waveguides with a quantum-confined Stark effect as high speed optical modulators [6-8]. Superlattice structures are expected to have a large absorption change because of the remarkable persistence of the exciton resonances at room temperature. It is also known that optical nonlinearities of semiconductors are greatly enhanced in superlattice structures over those of bulk crystals due to quantum size effects [3-5]. Low-loss short-wavelength optical waveguides composed of ZnSe/ZnS SLSs grown by low pressure MOVPE [21] was already reported. In order to realize the optical modulator and other new-functional devices using ZnSe/ZnS SLSs, it is important to investigate the effect of an electric field on the optical properties of ZnSe/ ZnS SLSs, including the nonlinear optical effect. ZnSe/ZnS SLSs and ZnS layers have been grown by low pressure MOVPE utilizing DMZ, DMSe and H2S. The substrates used are boatgrown Si-doped ( 1 0 0 ) GaAs (n = 1018cm-3). Photoluminescence (PL) measurements at 77 K were carried out to investigate the effect of an electric field on the emission properties. A H e Cd laser (325 nm, 10 mW) was used as an excitation source. For PL measurements, the layer structure consisted of a ZnS cladding layer (400 nm) on a GaAs substrate, a ZnSe (3 nm)/ ZnS (5 nm) (40 periods) SLS guiding layer and an upper indium-tin oxide (ITO) electrode which is transparent to the H e - C d laser and to the emission from the SLS. For n-GaAs, In/Sn metal was used as the electrode. Optical modulators using ZnSe/ZnS SLS waveguides were also fabricated. The modulator, which consists of a ZnSe (8 nm)/ZnS (8 nm)/50 periods) SLS guiding layer sandwiched by ZnS cladding layers (400 nm), was characterized using

109

T. Yokogawa I On ZnSe/ZnS and ZnTelZnS strained-layer superlattices

an Ar ÷ laser beam at 458 nm. Au/Sn electrodes were evaporated on both sides of the substrate. The electrodes were alloyed in the conventional manner. The wafer was cleaved into the waveguide length of 1 mm. The break-down voltage was about 2 V. Figure 10 shows the effect of electric field on the PL of ZnSe (3 nm)/ZnS (5 nm) (40 periods) SLS at 77 K. The SLS shows intense excitonic emission at 3.005 eV. Significant quenching of the PL intensity is observed as well as a shift in the PL energy with increasing reverse bias voltage. Figure 11 shows the intensity and the peak position of the excitonic emission as a function of the applied voltage. The PL intensity decreases and the PL peak position shifts toward the lower energy side with increasing electric field. Since the photocurrent of 1 p,A observed at 77 K is much smaller than the effective excitation current (about 260 p,A) estimated from the He-Cd laser power (10 mW) and the absorption in the superlattice, the dependence of PL intensity is not due to field-induced carrier leakages. Therefore, the quenching of excitonic emission is caused by the spatial separation of carriers

77K

°

>-

[

A

/ V e x t :0 V

W

z Z

V

-

t~

m W W

v I

[

380

I

I

~00

WAVELEN6TH

( n m)

I

I

420

Fig. 10. Photoluminescencespectra for various reverse-bias voltages at 77 K. The sample was a ZnSe(3 nm)/ZnS(5 nm) (40 periods) strained-layersuperlatticegrownon a ZnS layer (400 nm).

to .0

>I--

3.010

Z IaJ I--

z 0.5 Z O 1./3 03 3r LU

3.005

>

>0 tv tU -

3.000 z Y LIJ I1-

I

I

~

I

1.995

0 -5 -10 -15 -~ APPLIED VOLTAGE(V) Fig. 11. Dependence of the exciton peak intensity and position on the reverse-bias voltage. induced by the electric field. As the electric field is applied perpendicular to the heterointerface, the electron and hole distributions are polarized in opposite directions [22,23]. Consequently, the radiative recombination probability is efficiently decreased, and the nonradiative to radiative recombination ratio is changed. The observed shift of PL peak position to lower energy is also interpreted as the field-induced electron-hole separation, combined with a perturbation of the energy levels of the excitons in the well produced by the field. Generally, luminescence quenching of excitons in bulk ZnSe induced by an electric field is interpreted on the basis of an exciton dissociation by impact ionization. An intense excitonic emission line is observed from SLSs subjected to electric fields as high as 140kV/cm (10V) although the electric field required to ionize an exciton in bulk ZnSe is calculated to be about 50kV/cm. It was already reported that the PL thermal quenching properties in SLSs strongly depend on the exciton binding energy which increases with decreasing ZnSe well layer thick-

110

T. Yokogawa I On ZnSe / Z n S and Z n Te / Z n S strained-layer superlattices

ness [20]. Taking these results together, it is concluded that excitons confined in SLSs are very stable even in high electric fields due to the large exciton binding energy. Electric-field induced absorption in ZnSe/ZnS SLS waveguides was investigated. The waveguides were characterized at a 458nm wavelength by coupling the beam of an Ar + laser through their cleaved edge. Figure 12 shows the typical output power as a function of the voltage applied to the ZnSe/ZnS SLS waveguides. The output power from the cleaved facet of the waveguide is monitored by a photodetector. Samples were selected with an absorption edge quite close to the incident light wavelength. This facilitates manifestation of the quantum confined Stark effect. The thicknesses of ZnSe wells and ZnS barriers were both 8 nm, which result in an absorption edge at 450nm. The waveguide length is 1 mm. It is shown in fig. 12 that the output power decreases with increasing reverse voltage. By repeating the experiments at incident wavelength of 458 nm, 488 nm and 633 nm, we found that the on/off ratio of the output at the same bias voltage gets larger as the incident light wavelength becomes closer to the absorption

rY IJJ

I

edge of SLSs. At a reverse bias voltage of 20 V, on/off ratios of 4, 1.7 and 1 were obtained at wavelengths of 458, 488 and 633 nm, respectively. Although the power attenuation depends on both the reverse bias voltage and the wavelength of the guided wave with respect to the absorption edge of the SLSs, the optical switching operation of the ZnSe/ZnS SLS modulators is confirmed for the first time. To show the optical nonlinearity of ZnSe/ZnS SLSs, SHG in ZnSe/ZnS SLS using either a YLF laser or an AIGaAs/GaAs semiconductor laser was investigated. The SHG conversion efficiency strongly depends on the crystalline orientation of the waveguide, that is, the orientation of the waveguide surface and the propagation direction of the guided wave. Using epitaxial growth techniques such as MOVPE or molecular beam epitaxy (MBE), any surface orientation can be obtained by selecting the orientation of the substrate. On the other hand, in the case of polycrystalline ZnS film prepared by sputtering or a similar method, only the (1 1 1) plane is formed regardless of the crystalline orientation of the substrate [24]. Thus, it is a great advantage of epitaxial growth that we can select the optimized surface orientation, which provides the maximum conversion efficiency. In the zinc blende structure, the second-order optical nonlinear polarization P is described by the following tensor:

Ex

0

2

Ey I:L I,-

P=

0

0

0

d

0 0 0 0

0 W

_J i,i rr

o

1 I I I I 0 -5 -10 -15 -20 APPLIED VOLTAGE (V)

Fig. 12. Variation in waveguide throughput with reverse-bias voltage. The output power from the cleaved facet of the waveguide was measured by a photodetector. The waveguide structure consisted of a ZnSe (8 nm)/ZnS (8 nm) (50 periods) strained-layer superlattice sandwiched between ZnS layers (400 nm).

I E~

(4)

2EyEz 2EzEx ,2ExEy.

where E is an electric field of the fundamental wave, and the subscripts x, y and z represent the crystal orientations (1 0 0), (0 1 0) and (0 0 1 }, respectively. The effective second-order nonlinear coefficients daf are calculated in the case of the surface orientation n = (0 0 1), (0 1 1) and (1 1 1). The dCf~ is defined as the n direction component of P divided by E 2, since the

T. Yokogawa I On ZnSe/ZnS and Z n T e l Z n S strained-layer superlattices

TE--~TM conversion is considered. The effective nonlinear coefficient also depends on the propagation direction of the incident wave. First, the case of n = (00 1) is considered. The propagating direction d is described by using a parameter 0, defined as the angle between the (1 0 0 ) direction and the d vector in the ( 0 0 1 ) plane, as shown in fig. 13(a). E has components E = (E sin 0, - E cos 0, 0).

(5)

From eqs. (3) and (4), P and d~ff are calculated as

sin 2 0 ) ,

P = ( 0 , O, - d E

(6)

SH Wove TM

n

,,1'

Z~

(OlO)

d~ff = d sin 20.

(7)

In the case of n = (01 1) and (1 1 1), deff is determined by similar calculations. The definition of 0 in each case is shown in figs. 13(b) and 13(c). Table 1 shows the calculated values of E, P and deff in the cases of n = ( 0 0 1 ) , (01 1) and (111). Figure 14 shows the effective second-order nonlinear coefficient d~ff as a function of the incident angle 0, for n = ( 0 0 1 ) , ( 0 1 1 ) and (1 1 1). This shows that the maximum d~ff is obtained with n = ( 0 0 1 ) and d -- ( 1 1 0 ) . In the zincblende structure, flat (1 10) vertical surfaces are easily obtained by cleaving; thus the optimized waveguide structure can be fabricated by the simple processing technique of growing on the (001)-oriented substrate and cleaving the (1 10) surface.

i.o --n:,O01>

(10o)

(001) F'undomentol Wove TE

.

(T1o)

3-(1001

..... n:<011> - . -

n= <111>

(a)

(o~I)

(011)

111

.

.

.

.

.

.

.

.

.

.

(112)

(111)

(b)

.

0

(c)

Fig. 13. Explanation of the direction of the incident light, assuming that the orientation of the waveguide surface is ( 0 0 1 ) (a), ( 0 1 1 ) (b) and ( 1 1 1 ) (c).

10 20 30 40 50 60 70 80 90 INCIDENT ANGLE e (degree)

Fig. 14. Effective second-order nonlinear coefficients as a function of the incident angle of the fundamental wave.

Table 1 Calculated values of E, P and d e . for n = ( 0 0 1 ) , (0 1 1) and (1 1 I). n

(001)

(0 11)

(1 1 1)

E E~ Ey

Esin0 - E cos 0

Esin0

1 / V ~ E sin0 + 1/V'EE cos0 1 / V 6 E sin O - 1 / V ~ E cosO - 2 V ~ E sin 0

E~

0

- 1/X/2Ecos 0 1/ V~E cos 0 - d E 2 cos20

P P~ Pr Pz

0 0 - d E 2 sin 20

V ~ dE 2 sin 0 cos 0 -V'EdE: sin 0 cos 0

- 2 ~ 3 d E 2 sin E0 + 2/X/3dE 2 sin 0 cos 0 - 2 ~ 3 d E Esin E0 - 2 / V ~ d E 2 sin 0 cos 0 1~3dEE sin 20 - dE Ecos 20

d©ff

d sin 20

0

1/V~d

T. Yokogawa / On ZnSe/ZnS and ZnTe/ZnS strained-layer superlattices

112

RT

RT

YLF

AIGoAs LD

E tO v >I--

z

LLI t.--Z N

I -l

521

i

LT

i

'l'

522 524 526 WAVELENGTH (nm)

I

I

420 430 440 450 WAVELENGTH (nrr0

Fig. 15. Typical spectra of second-harmonic waves obtained in the ZnSe (2,5 n m ) / Z n S (5 nm) (100 periods) strained-layer superlattice. The fundamental wavelengths are the YLF laser line at 1047 nm and the A I G a A s / G a A s semiconductor laser line at 880 nm.

SHG in the SLSs is investigated by using a Q-switched YLF laser (A = 1047 nm, pulse peak power 600 W) or a AIGaAs/GaAs semiconductor laser (A=880nm, CW 35mW) as a fundamental wave. A green and blue SH wave was observed in the SLS layer, although the optimum phase matching condition was not achieved in this sample. Typical spectra of the observed SHG are shown in fig. 15. It is seen that the peak wavelength is located at 523.5 and 436 nm, and corresponds to the second harmonic of the input. The frequency doubling using SHG in II-VIs is useful for obtaining short-wavelength coherent light.

5. ZnTe/ZnSTe doping superlattice The doping superlattice or type II superlattice represents a new class of semiconductor material with unusual optical properties that rise from a very effective spatial separation of electrons and holes (indirect gap in real space) [25-27]. This separation implies two characteristic properties: (1) the effective bandgap is not a constant material parameter but is tunable by a varying

the nonequilibrium carrier concentration; (2) the recombination lifetime of excess carriers is increased. The superlattice system ZnTe/ZnSTe is such a type II superlattice in which electrons and holes are located in different semiconductors. The 5 nm-ZnTe/5 nm-ZnSTe (100 periods) SLs were grown on 1 Ixm ZnSTe buffers on InP substrates by low-pressure MOVPE which produces abrupt heterointerfaces. DMZ, diethyltelluride (DETe), trimethylantimonide (TMSb) and HES were used as source materials. Antimony impurity (Sb) was selectivity doped in the ZnTe layers. The typical growth temperature was 450°C. The substrate was Fe-doped semiinsulating (1 0 0) InP wafer. The oxide removal has been carried out in a solution of 5 H2SO 4 : 1 H:O 2 : 1 H:O) before growth. The optical properties were characterized by cathodoluminescence (CL) measurements at room temperature and PL measurements at 77 K. ZnSxTel_ x (0~
65I\

113

T. Yokogawa / On ZnSe/ZnS and ZnTe/ZnS strained-layer superlattices 3.5

ZnSxTel.x v

m 3.0r5 r.5 6.0 Z

2.5-

0

InP 5.5

0

~

0.i

*

I I I~ I I \ I 0.i 0.2 0.3 0.4 0.5 0.6 0.7 S COMPOSITION X

Fig. 16. Lattice constant of a ZnSTe layer perpendicular to the interface as a function of alloy composition. The lattice constants were measured by double crystal X-ray diffraction. The sulfur compositions x of the ZnS,_,Te, layers were determined by X-ray microanalysis.

ments at r o o m temperature. Figure 17 shows the emission energy as a function of the sulfur content x of the ZnSxTel_ x layer. The variation of the energy with alloy composition is nonm o n o t o n i c and concave The bandgap of ZnS0.35Te0.65 which is lattice-matched to InP is about 2.2 eV. Next, the Sb impurity doping of the Z n T e layer was investigated. The atomic depth profiles of Te and Sb in Sb-doped Z n T e layers are revealed by SIMS measurements, as shown in fig. 18. N o surface segregation of Te is observed, and the doping profile of Sb impurity in Z n T e layer is extremely uniform except near the heterointerface between Z n T e and InP. The Sb concentration definitely increases with increasing d o p a n t concentration in the gas phase. Z n T e / Z n S T e doping SLs were grown on 1 ~ m thick Z n S T e buffer layers on InP substrates by M O V P E . The SLs consist of 100 alterations of 5 nm Z n T e and 5 nm ZnSTe. The sulfur composition x of the constituent ZnSxTel_ x layer is inferred to be 0.35 from the X-ray microanalysis

0.2

0.3

0.4

COMPOSITION

0.6

0.5 X

Fig. 17. Emission energy versus sulfur composition x for ZnSl_xTex at room temperature. The emission energies were obtained from cathodoluminescence measurements at room temperature. 10 6 Te

Z

I05

© o

10 4

TMSb FLOW

Z ©

~'~

;;--

>~ 10 3 < ~l 10 2 Z © Q~

RATE

2.3 (×10 "5 mol/mi~.j

i06

iF

"~$b

10 ~ I

0

2JO

4'0 6'0 810 100 DEPTH (arb. units)

120

Fig. 18. Depth profiles of a host Te atom and Sb impurity in Sb-doped ZnTe by SIMS measurement. Sb depth profiles were obtained from three samples grown with various dopant concentration in the gas phase as indicated. of thicker single epitaxial layers grown with the same flow rates, pressure and substrate t e m p e r a ture. Sb was selectively doped in the constituent Z n T e layers. The Sb-doped Z n T e layers were grown with the same conditions to obtain the high hole concentration of 1 x 1018cm -3. PL m e a s u r e m e n t s at 77 K were carried out. Figure

T. Yokogawa / On ZnSe/ZnS and ZnTe/ZnS strained-layer superlattices

114

The properties of ZnTe/ZnSTe doping SLs grown on InP substrates by MOVPE have been investigated. The optical properties of the ZnTe/ ZnSTe doping SL are very useful for new functional optoelectronic devices in the visible region such as photonic switches.

250 W/cm2

.6 6. Electroluminescence in p - n junctions using p-ZnTe / ZnS doping superlattices o3

7~ b~

z

100 W/cm 7

25 W/¢m2 _ I 500

550

% I 600

65O

W A V E L E N G T H (nm) Fig. 19. Photoluminescence spectra of ZnTe/ZnSTe doping superlattices for various . . exc~tatmn . . .densxtles of the Ar + laser. These spectra were measured at 77 K.

19 shows the PL spectra in ZnTe/ZnSTe doping SLs for various excitation densities of Ar ÷ laser. A PL peak at 2.03eV is observed at lower excitation density (25 W/cm2), which shifts toward higher energy as the excitation density increases. At the high excitation density of 250 W/cm 2, the PL peaks at 2.12 eV. This result provides an experimental confirmation that the dominant luminescence process is the recombination of electrons in the conduction bands with holes in the acceptor impurity band, across the indirect gap in real space [27]. The thermalization of photoexcited electrons and holes is known to be a fast process compared to the electron-hole recombination. This means that most of the photoexcited carriers in ZnTe/ ZnSTe SL will be separated in real space before they have a chance to recombine via a vertical transition in real space. As a consequence, the photon energies of radiative recombination between thermalized electrons and holes, which reflects the effective energy gap, depend strongly on the concentration of photoexcited carriers.

Wide band gap II-VI semiconductors are promising materials for blue LEDs and LDs [1,2,31,32]. Using the MBE technique with a RF plasma source, blue LEDs and LDs have recently been fabricated. However, because of the difficulties in growing low-resistive p-type ZnSe and ZnS, there are few reports on blue electroluminescence (EL) in p-n junction LEDs using MOVPE. ZnTe is the only wide band gap II-VI semiconductor presenting p-type conductivity [33], although its band gap energy of 2.26 eV does not correspond to blue light but to green light [33-36]. The SL structure has attracted much attention due to the quantum size effect, which produces a new type of band structure and strong resonance of excitons [20,33,37,38]. A doping superlattice consisting of p-ZnTe and ZnS which presents p-type conduction as well as the wide band gap in the blue region was proposed earlier [39]. Here, for the first time, p-n junction blue LEDs using pZnTe/ZnS doping SLs are proposed and successfully demonstrated. Figure 20 shows that the structure of the p - n junction diode comprises p-type doping SLs and n-type ZnS. The p-type doping SLs consist of 100 alternations of Sb-doped 2 nm ZnTe and undoped 2 nm ZnS. The electrode for the SLs takes the form of Au dots of 1 mm diameter deposited onto the top SL layers through a metal mask. The top layer of the SLs is highly doped p-ZnTe. Ohmic contact to the back surface of the n-ZnS substrate is obtained using I n : H g alloy. The substrates are annealed at 440°C for 40 s to obtain ohmic contacts. P-type doping SLs were grown by MOVPE using DMZ, DETe and H2S. Reactants were

T. Yokogawa / On ZnSe/ZnS and ZnTe/ZnS strained-layer superlattices

115

2 nrn-ZnTe:Sb/2 nrn-ZnS p-type doping SL

i x l O Is

(Hole conc. p - 1 x 1017 cm "3)

/ 100

periods

#" lX1017

2

** In:Hg n-type Zn5:I substrate (electron conc. n--1 x 1016 cm "3)

Fig. 20. T h e structure of the p - n junction light-emitting diode.

separately introduced into the reactor with purified H 2 cartier gas at a flow rate of 2 l/min. The substrates were ZnS crystals grown by the iodine-transport method at 850°C. The ZnS substrates were annealed in molten Zn at 950°C to obtain low-resistive n-type material (1× 1026 cm-3). P-type doping SLs were grown on the (1 1 0) cleaved surface of ZnS substrates. The typical growth temperature and pressure were 450°C and 75 Torr, respectively. The impurity Sb was selectively doped into the ZnTe layer of SLs by using TMSb. Typical flow rates of DMZ, DETe, H2S and TMSb were 40, 5-100, 6-40 and 6-301xmol/min, respectively. The superlattice structure was confirmed by observing ---first-order satellite peaks in the X-ray (4 0 0) diffraction pattern. The SL period calculated from these satellite peaks is 3.6 nm, which agrees well with the thickness estimated (4 nm) from the growth rates. The p-type doping characteristics were investigated for the ZnTe, ZnS0.35Te0.65 and ZnTe/ ZnS SLs. Hall measurements at room temperature were carried out in p-type layers with Au ohmic contacts. The substrates used here are InP. Figure 21 shows the hole carrier concentration as a function of dopant concentration in the gas phase. When Sb is selectively doped into the ZnTe layer of SLs at the TMSb flow rate, as shown in fig. 21, a sheet hole carrier concentration of 5 × 1012 cm -2 is obtained, which corre-

AZn~Te..SblZnS DopingSL

r.r.l ~ 1×10 le 0 r~ ~u

o i x l O Is

ZnS0.35Te0.65 o o

o

I

0 I TMSb FLOW RATE

1

I

2 3 (×10-Smol/min)

Fig. 21. Hole concentration vs flow rate of trimethylantimonide for Sb-doped Z n T e , Z n S T e and p - Z n T e / Z n S doping superlattice.

sponds to an average concentration of about 1 x 1017 cm -3 On the other hand, the maximum concentration in the ZnS0.35Te0.65 layer is only 1 × 1015 cm -3 at the same TMSb flow rate. The hole carrier concentration in the ZnTe layer and the ZnTe/ZnS doping SLs monotonically increases with the flow rate of TMSb. This result shows the good controllability of the electrical property in both the Sb-doped ZnTe layer and the ZnTe/ZnS doping SLs. The ZnTe single layer has a high carrier concentration of 1 x 1018 cm -3 by Sb doping. The current-voltage (I-V) characteristics at room temperature are shown in fig. 22. These diodes exhibit good rectification properties. The turn-on voltage is about 2.5 V. The reverse bias breakdown exceeds 10 V. Most diode structures of p-n ZnSe layers grown on GaAs substrates have been reported to have a large turn-on voltage [3,31,32,40]. This large voltage is attributed both to the high-resistive layer caused by misfit dislocations at the GaAs-ZnSe interface and to a large potential barrier at the Au electrode which blocks the transport of carriers

T. Yokogawa / On ZnSe/ZnS and ZnTe/ZnS strained-layer superlattices

116

r~

RT

>'

z

1 2 mAldiv.

a I--4

z

2 V/div.

2;

nce/

O

100

I 400

10 -1

< 1 0 -2

sI

I

I

1

I

450 500 550 WAVELENGTH (nm)

600

Fig. 23. Electroluminescence spectrum of the p - n junction light-emitting diode and photoluminescence spectrum of the p-ZnTe : Sb/ZnS doping superlattice at room temperature.

10 -3

c~ C_)

1 0 -4 lO-S

< ~: c~ ©

10 -s 1 0 -7

10 4 2

I

I

3

4

(459 nm). The peak position of the EL agrees well with the PL peak position in the SL. According to the common cation rule, the valence band offset (AEv) is much larger than the conduction band offset (AEc) in the ZnTe/ZnS heterostructure, which results in the injection of electrons into the SLs region. For both reasons, the EL is attributed mainly to the electron-hole recombination in ZnTe/ZnS doping SLs. The 103

FORWARD BIAS (V) Fig. 22. Current-voltage characteristic of the p - n diode.

to the junction. We have used ZnS crystals as substrates and Au electrodes on the highly doped ZnTe layer. This indicates low dislocation density at the substrate interface and no blockage to transport at the interface between the ZnTe layer and the Au electrode, which results in a small turn-on voltage. We obtained a diode ideality factor n = 1.3 for a forward-bias current up to 10 -3 A. Electroluminescence (EL) spectra for the typical LED was investigated at room temperature. Figure 23 shows the EL spectrum for the p-n diode and photoluminescence (PL) spectrum for the p-ZnTe:Sb/ZnS doping SL. The forward bias voltage of the diode is 4 V. The EL spectrum consists of a peak centered in the blue at 2.7 eV

4 10 2 _

0

~r,~ 101

-/

10 o

I 00

I

I

I

i01

CURRENT

I 10 2

(mA)

Fig. 24. Total emission intensity of electroluminescence at room temperature as a function of diode current.

T. Yokogawa / On ZnSe/ZnS and ZnTe/ZnS strained-layer superlattices

EL peak slightly shifts toward the higher energy side with increasing diode current. This optical property may arise from band bending in the type II ZnTe/ZnS SLs [33]. The total emission intensity of EL at room temperature was measured as a function of diode current. Figure 24 shows emission intensity as a function of diode current. The emission intensity increases linearly with the current. This result shows the low density of nonradiative recombination centers in the p - n junction. The blue emission appears bright to the eye at the edges of the circular gold dots. The present study sheds light on blue LDs using II-VI semiconductors.

7. Summary In this paper, nonlinear optical properties and application to short-wavelength optical devices in ZnSe/ZnS and ZnTe/ZnS strained-layer superlattices have been described. Growth of ZnSe/ ZnS strained-layer superlattices on CaF 2 substrates has been demonstrated, and the excitonic properties have been investigated. Transmission spectra in ZnSe/ZnS superlattice show two excitonic absorption peaks due to the transitions between n = 1 electron and heavy hole (Eo_hh) and between n = 1 electron and light hole (E~_lh). The energy separation between Ee_hh and Ee_ m increases with decreasing ZnSe layer thickness, which shows the increase of strain in the ZnSe well layer. The relationship between ZnSe well layer thickness and absorption and PL peak energy shows the quantum size effect. The growth of high-quality ZnSe/ZnS SLSs on CaF 2 is very useful for Cerenkov-type SHG devices. Interdiffusion in ZnSe/ZnS SLSs induced by low-damage N + or Li + ion implantation and low-temperature annealing has been investigated, and a large refractive index change, which is useful for waveguiding applications, has been demonstrated. Secondary-ion mass spectrometry analyses show that the periodic depth profile of the superlattice structure disappear after ion implantation and subsequent thermal annealing at 400°C. A significant decrease in the refractive

117

index is observed in the SLS which is implanted and annealed. PL measurements show that, after annealing at 400°C, strong excitonic emission around 400 nm is observed in implanted SLSs corresponding to the damage recovery. With increasing annealing time, the PL peak for an implanted SLS shifts towards the higher energy side, which clearly indicates layer disordering of the SLSs. This low-temperature planar process will be very useful for the fabrication of II-VI semiconductor optoelectronic devices. The effect of an electric field on the emission properties of ZnSe/ZnS SLSs has been investigated. With increasing electric field, the PL intensity decreases and the PL peak position shifts toward the lower energy side, which is due to the spatial separation of carriers induced by the electric field. Optical modulators using ZnSe/ZnS SLS waveguide were fabricated and the optical switching operation of the ZnSe/ZnS SLS optical modulator confirmed. Second harmonic generation is observed in ZnSe/ZnS SLSs by using YLF laser or AIGaAs/GaAs semiconductor laser as a fundamental laser. Coherent light at a peak wavelength of 523.5 or 436 nm was obtained from ZnSe/ZnS SLSs. The nonlinear optical properties of ZnTe/ ZnSTe doping SLs grown on InP substrates by MOVPE has been investigated. The Sb impurity is selectively doped in the constituent ZnTe layers of SL. With increasing excitation density of Ar ÷ laser, the PL peak observed in the ZnTe/ ZnSTe doping SL at 77 K shifts toward higher energy. From this result, it is confirmed that the dominant luminescence process is the recombination of electrons in the conduction bands with holes in the acceptor impurity band across the indirect gap in real space. The optical properties of the ZnTe/ZnSTe doping SL are very useful for new-functional optoelectronic devices in the visible region such as photonic switches. p-n junction blue LEDs using p-ZnTe/ZnS doping SLs have been demonstrated. These LED structures emit blue light at room temperature. These heterostructures provide p-type conduction and a sheet hole concentration of 5 × 1012cm -2 which corresponds to the average concentration of about 1 x 1017 cm -3. The pres-

118

T. Yokogawa / On ZnSe/ZnS and ZnTe/ZnS strained-layer superlattices

ent study sheds light on blue LDs using II-VI seminconductors.

Acknowledgements The author is grateful to T. Saitoh, E.L. Wolak, T. Narusawa and T. Onuma for valuable discussions. The author also gratefully acknowledges Professor J.L. Merz at University of California, Santa Barbara, for continuous encouragement and discussions.

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[18] E.D. Palik, in: Handbook of Optical Constants of Solids, ed. E.D. Palik (Academic Press, Orlando, FL, 1985) p. 429. [19] B.J. Skromme, M.C. Tamargo and J.L. de Miguel, Appl. Phys. Lett. 53 (1988) 2217. [20] T. Yokogawa, H. Sato and M. Ogura, J. Appl. Phys. 64 (1988) 5201. [21] T. Yokogawa, M. Ogura and T. Kajiwara, Appl. Phys. Lett. 52 (1988) 120. [22] E.E. Mendez, G. Bastard, L.L. Chang, L. Esaki, H. Morkoc and R. Fischer, Phys. Rev. B 26 (1982) 7101. [23] Y. Kan, M. Yamanishi, I. Suemune, H. Yamamoto and T Yao, Japan. J. Appl. Phys. 24 (1985) L589. [24] H. Ito, N. Uesugi and H. Inaba, Appl. Phys. Lett. 25 (1974) 385. [25] K. Ploog, A. Fischer and H. Kunzel, J. Electrochem. Soc. 128 (1981) 400. [26] H. Kunzel, G.H. Dohler, P. Ruden and K. Ploog, Appl. Phys. Lett. 41 (1982) 852. [27] H. Jung, G.H. Dohler, H. Kunzel, K. Ploog, P. Ruden and H.J. Stolz, Solid State Commun. 43 (1982) 291. [28] S. Larach, R.E. Schrader and C.F. Stocker, Phys. Rev, 108 (1957) 587. [29] R. Hill and D. Richardson, Thin Solid Films 15 (1973) 303. [30] R. Hill and D. Richardson, J. Phys. C 6 (1973) Ll15. [31] J. Ren, K.A. Bowers, B. Sneed, D.L. Dreifus, J.W. Cook Jr. and J.F. Schetzina, Appl. Phys. Lett. 52 (1990) 1902. [32] R.M. Park, M.B. Troffer, C.M. Rouleau, J.M. DePuydt and M.A. Haase, Appl. Phys. Lett. 57 (1990) 2127. [33] T. Yokogawa and T. Narusawa, J. Cryst. Growth 117 (1992) 480. [34] M. Kobayashi, N. Mino, M. Konagai and K. Takahashi, Appl. Phys. Lett. 48 (1986) 296. [35] M. Ekawa, Y. Kawakami, T. Taguchi and A. Hiraki, J. Cryst. Growth 93 (1988) 667. [36] F.S. Turco-Sandroff, R.E. Nahory, M.J.S.E Brazil, R.J. Martin and H.J. Gilchrist, Appl. Phys. Lett. 58 (1991) 1611. [37] T. Yokogawa, T. Saitoh and T. Narusawa, J. Cryst. Growth 101 (1990) 550. [38] T. Yokogawa, T. Saitoh and T. Narusawa, Appl. Phys. Lett. 58 (1991) 1754. [39] T. Yokogawa and T. Narusawa, in: IEEE International Electron Devices Meeting, Washington, DC, December 8-11, 1991, Tech. Dig., p. 627. [40] M.A. Haase, H. Cheng, J.M. DePuydt and J.E. Potts, J. Appl. Phys. 67 (1990) 448. [41] T. Saitoh, T. Yokogawa and T. Narusawa, Appl. Phys. Lett. 55 (1989) 735.