Vertical-cavity electrically driven lasers with three GaInAs

quantum wells and diameters of several μm exhibit room-temperature pulsed current thresholds as low as 1.3mA with 958 nm output wavelength

Florez, L. T.

Harbison, J. P.

Jewell, J. L.

Lee, Y. H.

McCall, S. L.

Scherer, A.

Walker, S.

English

Caltech Authors - Main

usual memory demand associated with matrix inversion tech- 
niques. 
J.-M. JIN 20th June I989 
J. L. VOLAKIS 
Radiation Lnboratory 
Department of Electrical Engineering & Computer Science 
Uniuersity of Michigan 
Ann Arbor, MI 48109, U S A  
References 
1 AUCKLAND, D. T., and HARRINGTON, R. F.: 'A nonmodal formulation 
for electromagnetic transmission through a filled slot of arbitrary 
cross section in a thick conducting screen', IEEE Trans. Microw. 
Theory Tech., 1980,2S, pp. 548-555 
2 AUCKLAND, D. T., and HARRINGTON, R. F.: 'Electromagnetic trans- 
mission through a filled slit in a conducting plane of finite thick- 
ness, TE case', IEEE Trans. Micro. Theory Tech., 1978, 26, pp. 
499-505 
3 WANG, N.: 'Electromagnetic scattering from a filled slit in a thick 
conducting screen, TM case'. The Ohio State Univ. ElectroScience 
Lab. Tech. Report 714349-8, May 1984 
LOW-TH RESHOLD ELECTRICALLY PUMPED 
VERTICAL-CAVITY SUR FACE- E M  llTl NG 
M IC RO LAS ERS 
Indexing terms: Semiconductor lasers, Quantum optics 
Vertical-cavity electrically driven lasers with three GaInAs 
quantum wells and diameters of several pm exhibit room- 
temperature pulsed current thresholds as low as 1.3mA with 
958 nm output wavelength. 
Vertical-cavity surface-emitting show promise for a 
variety of applications. The lowest threshold edge-emitting 
lasers contain a single quantum well and require approx- 
imately 0.55 mA for operation.'6 Optically pumped vertical- 
cavity lasers with pm dimenions' have shown promise for 
much lower thresholds, especially if only a few, or even one, 
quantum well is used.' 
We have constructed a large number of electrically pumped 
vertical-cavity surface-emitting semiconductor lasers with 
dimensions of a few pn (p-lasers) on a single GaAs chip. 
Diameters are 2, 3, 4 and 5pm and heights are 5.5pm. The 
yield is between 95 and 100% for 5 pm diameter devices in all 
examined regions. Pulsed thresholds range from 1.3 mA (3 pm 
diameter) to 2mA (5pm diameter). The device density is about 
two million per cm2. The chip size is about 3 x 5 mm. 
Molecular beam epitaxy was used to grow the sample. The 
substrate was Si-doped n+-GaAs on which alternate layers of 
AlAs and GaAs were grown, forming an interference mirror. 
These layers, each nominally a 1/4-wave optical thickness at 
980nm, were Si-doped at 3 x lO ' '~ rn -~ .  The undoped active 
region consists of three 80 A Ga,.,In,.,As quantum wells 
interleaved with four 100 A GaAs barriers. Surrounding the 
active region are 200 A compositionally graded' undoped 
AI,Ga,-,As layers with x ranging from 0 to 0.5. Above and 
below the graded layer active region complex are spacer 
regions of doped AI,.,Ga,.,As of thicknesses so that a full 
wave exists between mirrors. The second mirror was con- 
structed similarly to the first using Be as the dopant. The 
innermost three layers of the second mirror, where the optical 
intensity is highest, were doped at 2.5 x 10'8cm-3, whereas 
the remaining outermost layers were doped at 5 x lO ' '~ rn -~ .  
The Si-doped mirror had 20 half-wave periods; the Be-doped 
mirror had 12. Above the Be-doped mirror were grown a 
-0.2 optical wave thickness of AlAs and a 30A GaAs cap of 
GaAs, both Be-doped at 5 x l O ' ' ~ n - ~ .  For low contact 
resistance, the cap was also delta-doped" with l O I 3  cm-3 of 
Be. The cap layer was covered with unalloyed Au, 1500A 
thick. 
Standard contact photolithography with liftoff was used to 
create circular spots of Ni, 1-5pm diameter, on the Au. 
Chemical beam ion-beam-assisted etching was used to etch 
through the Au and the 5.35pm heterostructure, leaving the 
Ni covered areas. The Ni thickness was chosen so that it 
barely eroded away. 
The other side of substrate was polished and a wire serving 
as ground was spot-welded to  it. Positive voltage pulses were 
applied to the Au caps using a small probe tip of radius 
-5pm. Laser outputs went through the substrate and out the 
polished side. Luminescence was unpolarised and the laser 
light was linearly polarised. 
Fig. 1 displays output light at 958nm against driving 
current for a 3pm p-laser. Voltage pulses were 5011s at low 
l n w t . m A  
Fig. 1 Output light power against current 
Differential quantum elliciency is about 16%. Inset shows typical 
spectrum at 1.6mA for a 3pm diameter microlaser. Peak full width 
at half-maximum is about 4 A, the spectrometer resolution. Trace 
spans from 9300 to 9845 A 
duty cycle. The measured single-facet differential quantum eff- 
ciency was about 16%, despite some absorption of the laser 
output in the doped substrate. At threshold the voltage was 
about 15V, rising to  20V at 3.5mA. The 5fim p-lasers 
required about 8 V  and 2mA for threshold. This very low 
1.3 mA current threshold was obtained despite very fast 
expected carrier recombination due to  surface recombi- 
nation." The fast carrier recombination and high mirror 
resistances caused substantial heating preventing room- 
temperature CW operation. No  heat-sinking was used. We did 
operate with duty cycle as high as 40%. The reduction of 
current thresholds would reduce both the voltage drop and 
device heating. The inset shows a spectrum taken using 5011s 
1.6mA pulses. During a single longer pulse operation, wave- 
length shifts of 25 A were observed, corresponding to  a 50°C 
temperature rise. Lasing wavelength variations of & 15 A were 
noted upon driving lasers on different portions of the chip. 
Fig. 2 displays the output pulse train obtained when a 
Fig. 2 Laser output light pulses when driven by continuous 230 M H z  
uoltage sine wave 
Detection system used Si avalanche photodiode and sampling 
oscilloscope. Observed l00ps rise time is approximately response 
time of system 
ELECTRONICS LETTERS 17th August 1989 V o l .  25 N o .  17 1123 
p-laser was driven with a continuous 230MHz voltage sine 
wave. 
Surface recombination should be much less important with 
GaInAsP compounds used for lasers with 1.55 pm wavelength 
output. The suppression of surface recombination” should 
allow current thresholds less than 10pA in smaller devices. 
Reduced threshold current, improved heat-sinking, or lower 
resistance mirrors should also allow room temperature CW 
operation. 
We thank R. Martin for the spot-welding to  the substrate. 
J. L. JEWELL 
A. SCHERER’ 
S. L. McCALLt 
Y. H. LEE 
S. WALKER 
J. P. HARBISON* 
L. T. FLOREZI 
AT&T Bell Laboratories 
Hofmdel, NJ 07733, USA 
* Bellcore 
Red Bank, NJ 07701, USA 
t AT&T Bell Laboratories 
Murray Hill, NJ 07974, USA 
19th June 1989 
References 
I KINOSHITA, s., MORITO, K.,  KOYAMA, F., and IGA, K . :  ’Reproducible 
fabrication of AIAs/GaAs circular buried heterostructure (CBH) 
surface-emitting lasers with low threshold‘, Electron. Lett., 1988, 
24, pp. 699-700 
2 SAKAGUCHI, T., KOYAMA, F., and IGA, K.: ‘Vertical cavity surface- 
emitting laser with AlGaAs/AIAs Bragg reflector’, Electron. Lett., 
1988,24, pp. 928-929 
3 KOYAMA, F., KINOSHITA, s., and IGA, K.: ‘Room temperature cw 
vertical cavity surface emitting laser and high power 2-D laser 
array’, Tech. Dig., CLEO ’89 (Optical Society of America, 
Baltimore), paper FCI 
ROSENBERG, J. : ‘Ultralow-threshold graded-index separate- 
confinement single quantum well buried heterostructure (AI, 
Ga)As lasers with high reflectivity coatings’, Appl. Phys. Lett., 
1987, SO, pp. 1773-1775 
5 LAU, K. Y., DERRY, P. L., and YAKIV, A . :  ‘Ultimate limit in low 
threshold quantum well GaAlAs semiconductor lasers’, Appf. Phys. 
Lett., 1988, 52, pp. 88-90 
6 YARIV, A.: ‘Scaling laws and minimum threshold currents for 
quantum-confined semiconductor lasers’, Appf. Phys. Lett., 1988, 
53, pp. 1033-1035 
and ENGLISH, J. H.: ‘Lasing characteristics of GaAs micro- 
resonators’, Appf.  Phys. Lett., 1988.54, pp. 1W1402 
L., and CHAO, A. Y.: ‘Vertical cavity single quantum well laser’, 
Appf. Phys. Lett., 1989 
9 TSANG, w .  T.: ‘Extremely low threshold AlGaAs graded index 
waveguide separate confinement heterostructure lasers grown by 
molecular beam epitaxy’, Appl. Phys. Lett., 1982,40, pp. 217-219 
10 XHUBERT, E. F., CUNNINGHAM, J. E., and CHIU, T. H.: ‘Delta-doped 
ohmic contacts to n-GaAs’, Appf. Phys. Lett., 1986, 49, pp. 
292-294 
11 JEWELL, J.  L., SCHERER, A., MCCALL, s. L., GOSSARO, A. c., and 
4 OERRY, P. L., YARIV, A., LAU, K. Y., BAR-CHAIM, N., LEE, K., and 
7 JEWLL,  J. L., MCCALL, S. L., LEE, Y. H., SCHEKER, A., GOSSAKD, A. C., 
8 JEWELL, 1. L., HUANG, K. F., TAI, K., LEE, Y. H., FISHER, R. J., MCCALL, S. 
ENGLISH, J. H.: ‘GaAs-AIAs monolithic microresonator arrays’, 
Appl. Phys. Lett., 1987,51, pp. 94-96 
12 SANOROFF, C. J., HEDGE, M. s., FARROW, L. A., CHANG, c. c., and 
HAKBISON, J. P.: ‘Electronic passivation of GaAs surfaces through 
the formation of arsenic-sulfur bonds’, Appl. Phys. Lett., 1989, 54, 
pp. 362-364 
H IG H - FR EQU ENCY PER FOR M A N  CE 0 F 
MOVPE npn AIGaAsIGaAs 
H ETERO J U N  CTlON BIPOLAR TRANS ISTO RS 
Indexing terms: Semiconductor devices and materials, Bipolar 
deoices, Transistors, Epitaxy 
Base doping densities near 10” and emitter doping 
densities near 7 x l O ” ~ m - ~  have been achieved in MOVPE 
HBT structures and combined with self-aligned processing 
resulting in fmx = 94GHz and f, = 45GHz. To our knowl- 
edge, these are the first MOVPE HBTs demonstrated to 
operate at millimetre-wave frequencies. 
Introduction: The heterojunction bipolar transistor is a prom- 
ising high-speed device.’-’ The realisation of this high-speed 
performance is dependent on the reduction of parasitic delays. 
These delays can be reduced with a combination of self- 
aligned processing which reduces extrinsic parasitics and high 
doping densities in the emitter, base and collector which 
reduce the contact, spreading and intrinsic resistances. 
Epitaxial  g rowfh:  The HBT structure was grown by MOVPE 
at 40Torr and 550°C. It consisted of a superlattice buffer, 
6000.4 1 x 10’9cm-3 Te-doped GaAs subcollector, 50008, 
5 x 10’6cm-3 Si-doped GaAs collector, 8008, 1 x 1020cm-3 
Zn-doped GaAs base, 200A undoped GaAs spacer, 2000.4 
7 x 10” cm-3 Te-doped AI,.,,Ga, 7 0 A ~  emitter including 
300.4 of emitter-base grading and 500.4 of emitter-contact 
grading and a loOo.4 1 x 10’9cm-3 Te-doped GaAs emitter 
contact layer. A detailed study on the growth of HBT struc- 
tures with these high doping densities will be presented else- 
where.’ 
Device fabricat ion:  A fully self-aligned dual liftoff process: 
which had generated the best performing MBE HBTs, was 
employed to fabricate MOVPE HBTs. Inasmuch as the same 
process was employed to fabricate both MOVPE and MBE 
HBTs, a direct comparison can be made between HBTs 
grown by these two different epitaxial technologies. The dual 
liftoff process is capable of generating small geometry HBTs 
(emitter width down to 1.2pm) with low device parasitics 
while maintaining a high throughput optical lithography. 
Critical processing steps of the dual liftoff process can be sum- 
marised as follows. 
After device isolation, a single photoresist pattern was used 
to define an etch-down to the base layer of a HBT structure, 
thereby establishing the active emitter area. The same photo- 
resist was used in masking the photon implantation for base- 
collector capacitance reduction and the base metal 
evaporation. Finally, the photoresist was used to mask a low- 
temperature-deposited dielectric (Photo-CVD Si’N,). The 
base metal and the dielectric were patterned by liftoff as the 
photoresist is dissolved. The resultant structure iiad base 
metal in base contact areas overlaid with a dielectric which 
protected both the metal and the sidewalls. This process was 
termed ‘dual liftoff because unwanted base metal and dielec- 
tric were lifted off concurrently. Following the liftoff, one can 
use noncritical alignment to  define an emitter contact pattern 
because the emitter metal can overlap the covered extrinsic 
base area or the base contact area without concern for 
emitter-base shorting. Contact to  the collector was made by 
etching a via hole through the uppermost layers down to the 
buried subcollector. Polyimide was used to form the interlayer 
dielectric and two levels of interconnect metal were provided. 
All lithographic work was carried out with an optical contact 
aligner. 
Device performance: The self-aligned dual lift off process has 
generated MOVPE HBTs with minimum device parasitics 
and excellent R F  performances. According to the transmission 
line measurement on fabricated MOVPE HBT wafers, the 
sheet resistance of the base layer was found to be 124R/sq, 
corresponding to a base layer doping density of 1 x lOZ0/cm3 
(as quantified by the Hall measurement). The base contact 
resistance was extrapolated to  be 5.1 x 10-’Rcm2. Making 
use of Te as n-type dopant, we were able to  reduce both 
emitter and subcollector resistances by increasing doping den- 
sities up to 1 x 10’9/cm3. The contact resistances of the emit- 
ters and subcollectors were measured to  be 4.7 x 10-’Rcm2 
and 5.3 x IO-’Rcm*, respectively. The measured sheet resist- 
1124 ELECTRONICS LETTERS 17th August 1989 Vol. 25 N o .  17 


Low-Threshold Electrically Pumps Vertical-Cavity Surface-Emitting Microlasers

GaAs-AIAs monolithic microresonator arrays’,

Reproducible fabrication of AIAs/GaAs circular buried heterostructure (CBH) surface-emitting lasers with low

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Low-Threshold Electrically Pumps Vertical-Cavity Surface-Emitting Microlasers

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