Grating-coupler-induced intersubband transitions in semiconductor multiple quantum wells

Grating-coupler-induced intersubband transitions in semiconductor multiple quantum wells

164 Surface GRATING-COUPLER-INDUCED MULTIPLE QUANTUM WELLS INTERSUBBAND TRANSITIONS Science 22X (1990) 164- 167 North-Holland IN SEMICONDUCTOR ...

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164

Surface

GRATING-COUPLER-INDUCED MULTIPLE QUANTUM WELLS

INTERSUBBAND

TRANSITIONS

Science 22X (1990) 164- 167 North-Holland

IN SEMICONDUCTOR

W.J. LI, B.D. McCOMBE

Intersuhband transitions induced in a simple Faraday transmission geometry by metallic grating couplers have heen studied in srverai GaAs/Al,,Ga,,As MQW samples lightly doped with donors and with well widths between 210 A and 320 r\. A sensitive pumping and probing technique was employed in which chopped visible pump light and spatially modulated infrared radiation are transmitted simultaneously through the MQW. The excess free electron density in the well created by -C 100 ,uW/cm’ of red light 15 estimated to be - IO’cm-’ per well, The measured E, - E,, energies are in good agreement with a simple 1 D model calculation. It i\ found that the grating coupling efficiency drops from 15% to 5% when the ratio of the transition wavelength to the grating period i\ increased from 1.5 to 3.6

1. Introduction Confinement of electron motion in semiconductor quantum wells leads to discrete quantized states for motion in the confinement direction with rree motion in the plane of the layer, the so-called confinement subbands. Since the first observation of electronic transitions between these states [l], there have been numerous experimental [2-51 and theoretical [6,7] studies of this quantum phenoInenon because of its infrared radiation range, large electric dipole moment and narrow bandwidth which make it suitable for applications in infrared (IR) detectors and/or sources. Indeed. narrow band 1R detection [X,9] and emission [lo] have been reported in GaAs/AlGaAs quantum well structures. Most of these studies involve heavily (modulation)-doped structures. I-Iowever, a detailed and systematic experimental investigation of light!~-doped quantum-confined systems is necessary in order to understand in detail the t~~9-6~2~~90/$0~,50 (North-Holland)

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behavior of electrons and holes in single-particle subband states. This information will provide not only basic physical insight, but also guidance toward the achievement of high efficiency, high sensitivity IR sources and detectors. Since the subband states are due to the electron motion perpendicular to the plane of the quantum well, the corresponding transitions are only allowed with an electric field component of light polarized in the same direction. Therefore special couphng techniques have to be employed, such as prism couplers [ll], Brewster angle orientation (BAO) [3,4]. and multiple internal reflection (MIR) [12]. In BAO the polarized infrared beam is incident at the Brewster angle on the sample and is strongly refracted to give rise to a small component of electric field pe~endicular to the plane of the QW’s. The coupling to the transition dipoles is very inefficient (5 9.4% [12]). In the MIR technique, the IR beam is focussed onto a 45 degree polished edge of the sample, the light totally re-

W.J. Li et ai. / Intersuhband transitions in semrconductor

fleets within the sample for several times before it passes out the opposite edge and reaches the detector. This technique not only provides a larger E-field component in the confinement direction but also gives stronger absorption due to multiple internal reflection. The drawbacks of this technique are the small region of light exposure limited by sample thickness and the necessity of good optical focussing elements. In our experiments we use metallic grating couplers, which were fabricated on the sample surface by standard photolithography and lift-off techniques. The grating scatters the incident light and produces new electromagnetic waves with different polarization including a substantial component along the confined direction [lS]. The grating coupler has been originally applied to MOS systems [13] to study intersubband resonances in electron inversion layers of silicon. The unique feature of this technique is that it produces a substantial component of electric field in the confinement direction (even with normal incidence) while allowing simultaneous pumping of the underlying MQW structure with visible light to create excess carriers in the wells. Grating couplers can enhance the quantum efficiency of a quantum well detector from 1% to 90% [8].

quantum wells

165

Table 1 Sample description Nominal

Doping

Sample

well width (A) Amoco728

Well center doped: 1 x 10” cm-’ 30 periods

210

Corn31 74

Barrier center doped: 4x101hcm-3 30 periods Well top edged doped: lx10’6cm-3 12 periods

250

Corn1270

320

and Fourier transformed to obtain the frequency spectrum. This spectrum is then normalized to a background taken with chopped IR light. The spectrum so obtained is a difference spectrum between light-on and light-off. It has been found that the primary effect of the LED is to neutralize donors in the wells which are ionized by compensating acceptors. At temperatures > 30 K, these donors produce excess free electrons in the wells.

3. Results and discussion 2. Sample preparation and experimental techniques Three Al,,Ga,,As/GaAs multiple quantum well samples grown by MBE have been investigated. The parameters of these samples are listed in table 1. The small amount of Si doping allows one to ignore depolarization shifts. The aluminum gratings of linewidth 4 pm and period 8 pm were fabricated directly on the surface of the sample. In order to see the weak intersubband transition signal, a sensitive pumping and probing technique has been employed in which a red LED with power < 100 FW is square-wave modulated at audio frequencies and is directed onto the sample together with IR radiation that is spatially modulated by the grating (see fig. 1). The transmitted light is detected by a Ge: Ga photoconductive detector at 4.2 K. The signal is lock-in amplified

A typical differential intersubband absorption spectrum is shown in fig. 2 (solid line) for sample $1 (210 A well width). The spectrum taken under similar experimental conditions from the adjacent

metal X

M QWs

z GoAs Fig. 1. The sample-grating structure field distribution near the grating pumping.

grating

AlGaAs

substrate

showing the schematic IR with simultaneous LED

W.J. Li el al. / Intersuhband

166

AM0728 B=O OT

WITHOUT WITH

transitrons in semiconductor

GRATING----

GRATING--

T=6OY

I80

WAVE NUMBER

[CM-'!

Fig. 2. E, - E,, intersubband transition with peak position at 235 cm-‘. Spectrum from sample without grating coupler is also shown for comparision. The monotonic tail is due to free electron absorption.

section of the same wafer without grating coupler is also shown for comparision (dashed line). The 0 + 1 intersubband transition peaked at 235 cm-’ is clearly identified. The subband separation is calculated to be 238 cm-’ according to a simple 1D model calculation with barrier height of 230 meV (60-40 rule), in very good agreement with experimental observation. The well widths used in the calculation are from photoluminescence and reflectivity measurements taken on a different section of the same wafer. The monotonically decreasing background is free electron absorption due the residual in-plane component of IR electric field. The results for the other samples are summarized in table 2. Agreement is generally within the combined experimental uncertainty. Cyclotron resonance experiments were performed under the same experimental conditions. The pumping-induced carrier density, obtained from the integrated intensity of CR absorption, is

Table 2 Summary

of results from experimental

measurements

of the order of lO”/cm’ per well. This density is used in a calculation of the peak absorption per well (with a Lorentzian line of width equal to that observed representing the subband oscillator) with the additional assumptions of complete polarization of the incident wave normal to the interface and electric field uniform over the MQW region. The efficiency is then defined as the ratio of the measured peak absorption to that calculated. Results are shown in table 2. The coupling efficiency shows a systematic decrease when X/u is in creased (here h is the transition wavelength and u is the period of the grating). This is consistant with other observations [lo]. When X/U > 1, the z-component of electric field is a decaying wave with penetration depth given by u 6= 2?7b”l- (u/X)’ . It is also likely that the efficiency will be reduced when 6 is significantly larger than the thickness of the MQW structure (as in the case of Corn 1270): but a conclusive result can not be inferred from the present samples since the total variation of the ratio of 6 to the MQW thickness is only 25W, and the efficiency does not show systematic behavour as a function of this ratio. In summary, we have observed intersubband transitions via metallic grating couplers from three lightly-doped MQW samples. The measured E, E,, energies are in good agreement with calculation. The grating coupling is found to decrease when h/a is increased. A detailed lineshape study will be presented in a future publication. Acknowledgements The authors thank Professor W. Anderson and Mr. E. Cheng for providing lithography facilities

and calculation x/a

Coupling efficiency

235 i_ 2

IS2

15%

170+

2

2.10

99

99 * 2

3.60

5%

Well width (A) (PL + Refl.)

E, -E, (cm-‘) (Calculated)

E, ~ E. (cm-‘) (ISB Exp.)

Amoco728

210 + 5

238 i 6

Corn31

74

240 + 5

181&4

Corn1 270

320 * 5

110*

Sample

quantum we//s

3

W.J. Li et al. / Intersubband irrlnsitions in semiconductor quantum wells

and technical assistance, and Mr. X.C. Liu and Professor A. Petrou for the PL and reflectivity measurements. This work was supported by AR0 and ONR/ SDIO.

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and L.Z. Liu, Appl. Phys. Lett. 47 (1985) 289. D.D. Coon, 0. Byungsung, Y.F. Lin [71 K.M.S.V. Bandara, and M.H. Frabcombe, preprint (1988). VI K.W. Goossen and S.A. Lyon, Appl. Phys. Lett. 47 (1985) 1257. 191 B.F. Levine, CC. Bethea, G. Hasnain. U. Walker and R.J. Malik, Appl. Phys. Lett. 53 (1988) 296. [lOI M. Helm, E. Colas, P. England, F. DeRosa and S.F. Allen, Appl. Phys. Lett. 53 (1988) 1714. [Ill B.D. McCombe, R.T. Holm and D.E. Schafer, Solid State Commun. 32 (1979) 603. and G. Landgren. preprint (1988). H21 J.Y. Andersson and U. Mackens, Phys. Rev. B 33 (1986) [I31 D. Heitmann 8269. in: Magnetospectroscopy P41 A. Petrou and B.D. McCombe, of Confined Semiconductor Systems, Landau Level spectroscopy, Eds. G. Landwehr and E.I. Rashba (North-Holland, Amsterdam, 1989) to be published. P51 T.W. Nee. Phys. Rev. B 29 (1984) 3225.