GaAs/AlGaAs Intersubband MID-Infrared Emitter
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has the smallest energy gap and largest refactive index and, thus, cannot be used as a lower cladding layer. In addition, AlGaAs with high Al-content has deep-lying donor states (DX centers), which give rise to inferior transport properties and could therefore induce a large series resistance. An alternative cladding concept is based on a short-period GaAs/AlAs superlattice. SAMPLES We report the growth and characterization of three different light emitting diodes described in table 1: sample #1 without any cladding layers, sample #2 with a highly doped superlattice cladding and sample #3 with highly doped thick AlAs-layers. sample number
substrate
lower cladding
active zone
top cladding
sample #1
S.I.-GaAs
none
25 periods
none
sample #2
S.I.-GaAs
n+-GaAs/AlAs superlattice
25 periods
n+-GaAs/AlAs superlattice
sample #3
n+-GaAs
n+-AlAs
25 periods
n+-AlAs
Tablel: sample description The MBE grown diode structures essentially follow the design considerations for a QC emitter given by Faist et al. in Ref.3 with the modifications necessary due to the other material system and consist of 25 periods of GaAs/A10 .45Gao. 55As coupled quantum well layers. 0.6
0.4
'*-njeCtor
.5E
0.16
0.5F
-7
0.32 r7ei
R
E
' . 'E'
S'E
0.08
0.2'
-E
0 El 400 500 600 700 800 900
0.1 900
1100
1300
Distance (A)
Distance (A)
Figure Ia: Calculated band structure of the Figure lb: Calculated band structure under active cell with injector (sample #1) bias: 70 kV/cm corresponds to Vb= 7.5 V The growth sequence of the active cell of samples #2 & #3 (#1) is: 10 A GaAs, 15 A AIGaAs, 47 (45) A GaAs, 22 (20) A AlGaAs, and 40 (45) A GaAs (see Fig. 1). After characterization of sample #1 the growth sequence of the active layers was slightly changed to suppress the direct 3-1 transition. The active cells are separated by doped (n = 1017 cm-3) miniband funnel
166
injectors, which provide effective injection through the tunnel barrier into the n=3 state and prevent the escape of electrons into the continuum. Tunnel barriers and active cells are left undoped in order to suppress the influence of doping impurities on the linewidth of the intersubband luminescence 16. The whole structure is embedded between two highly doped cladding or n+GaAs contact layers (table 1). When an electric field of 70 kV/cm is applied to the structure, which corresponds to the calculated operating bias of Vb=7.5 V, the separation of the n1l and n--2 subbands is around 35-40 meV. This energy difference ensures fast depopulation of the n=2 level due to efficient optical-phonon emission, which is required for population inversion and potential lasing. The separation between the n=2 and n=3 levels, which determines the emission wavelength, is calculated to 1510 cm- 1, (corresponding to a wavelength of 6.6 Rm). The present choice was dictated by two considerations: on the one hand, shorter wavelength emission (or lasing) is generally easier to achieve due to the larger spontaneous photon emission rate and the smaller optical-phonon emission rate, whi
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