Lateral Carrier Injection with n-type Modulation-doped Quantum Wells in VCSELs
Chin-Han Lin1, Yan Zheng1, Matthias Gross3, Mark J. W. Rodwell1, and Larry A. Coldren1,2
1
Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106, USA
2
Department of Materials, University of California, Santa Barbara, CA 93106, USA
3
Ziva Corporation, San Diego, CA 92121, USA
Phone: 1-805-893-7065 Fax: 1-805-893-4500 Email: chinhan@ece.ucsb.edu
We have demonstrated a novel Field-Induced Charge-Separation Laser (FICSL) in a Vertical-Cavity
Surface-Emitting Laser (VCSEL) embodiment. In addition to the initial optical modulation results that have been
presented [1], we here for the first time present details on the novel lateral charge injection structure as well as the
advanced bandgap engineering involved in the gate structure. These features together permit high-speed light
modulation with a nearly constant injection current. The result is an entirely new concept for high-speed
directly-modulated semiconductor lasers.
In conventional diode VCSELs, carriers are typically injected into the quantum wells from the perpendicular
direction (Fig. 3a-b). The common epitaxial design is a SCH (Separate Confinement Heterostructure) with p-type
and n-type DBR mirrors on the opposite side of the active region. On the other hand, to realize direct modulation
via a gate voltage, any highly doped intra-cavity contact layer above the quantum wells such as in Fig. 3b would pin
the Fermi level, screening the driving force from the gate. Hence, carriers have to be injected laterally into the
active region (Fig 3c). To allow only hole movement and isolate the electrons between the active channel and the
off-state well, the first period of the n-DBR has to be band-gap engineered to form a quantum barrier (Fig. 4).
Lateral injection of carriers into quantum wells has been demonstrated on lateral current injection (LCI) lasers.
However, the sheet resistance in LCI lasers is typically 2-3 times higher than a vertically-injected ridge laser with
the same dimensions [2]. In a VCSEL structure, the comparatively small dimensions makes the sheet resistance
even more detrimental, especially when there has to be a finite setback distance d between the current aperture edge
and the metal ring contact to avoid absorption loss (Fig. 1). To enhance the conductivity of lateral injection,
delta-doping can be applied to the quantum well barriers to provide free carriers.
Early research showed that modulation doping was promising for quantum well performance [3]. Furthermore,
both n-type and p-type doped quantum wells have been incorporated into diode lasers and demonstrated the
reduction of threshold current density [4, 5]. However, doping near the optical standing wave E2 peaks has to be
treated carefully since the increase in absorption loss and dopant scattering may compromise overall performance,
rendering the delta-doping benefit marginal.
To examine lateral injection with two-dimensional electron gas (2DEG) and two-dimensional hole gas (2DHG),
three In0.2Ga0.8As/GaAs quantum well samples were grown by Molecular Beam Epitaxy with different delta-doping
level in the barriers; silicon was used for n-doping, and carbon via a CBr4 source was used for p-doping. The sheet
charge density and the carrier mobility of these samples were acquired by room temperature Hall measurement, and
the overall sheet conductivity was calculated (Fig. 5). It is clear that due to the large difference in mobility, the
conductivity of 2DEG is more than one order of magnitude higher than a 2DHG with the same sheet charge density.
Material gain of the modulation-doped quantum wells was characterized by fabricating broad area lasers. It was
confirmed that both n-type and p-type modulation-doping reduced the transparency current density, while p-type
doping also increased the differential gain, dg/dJ (Fig. 6).
Due to the large difference in conductivity, n-type modulation-doped quantum wells are chosen to form the
active channels. FICSLs were fabricated and the measurement results are shown in Fig. 7. The required gate
voltage swing was higher than expected, indicating that the fabricated FICSLs had very large resistance in the
n-DBR, causing a large voltage drop. The large n-DBR resistance also limited the small-signal bandwidth to be
around 11GHz [1]. The differential series resistance between injector and channel was measured to be 550Ω in a
5µm (aperture diameter) device.
This work was supported by DARPA through a STTR with Ziva Corp. and by NSF through a GOALI program.
A portion of this work was done in the UCSB nanofabrication facility, part of the NSF funded NNIN network.
[1] C.-H. Lin et al., International Semiconductor Laser Conference (ISLC) 2010, post deadline paper, PD2, September 2010.
[2] E.H. Sargent et al., Journal of Lightwave Technology, Vol. 16, No. 10 (1998)
[3] K. Uomi, Japanese Journal of Applied Physics, Vol. 29, No. 1, pp. 81-87 (1990).
[4] T. Mukaihara et al., Physica B, Vol. 227, Issue 1-4, pp. 400-403 (1996).
[5] N. Hatori et al., Electron. Lett., vol. 33, pp. 1096–1097 (1997).
[6] L. A. Coldren and S. W. Corzine, Diode Lasers and Photonic Integrated Circuits, Chapter 4, Wiley, NY (1995).
978-1-61284-244-8/11/$26.00 ©2011 IEEE
259
Band edge energy (eV)
Gate
BCB
Active region
Channel
p-DBR
i-DBR
Semi-insulating GaAs substrate
3
2
(a)
200
400
Off-state
600
800
0
0
1000
Position (nm)
n-gate
(c)
SiN sidewall
n-channel
nDBR
highly doped p-layer
Al2O3
QWs
QWs Al O
2 3
@RT
n-type
δ-doping in the barriers
Al0.3Ga0.7As SCH
GaAs Barrier (80 Å)
-3
10
In0.2Ga0.8As Well (80 Å)
p-type
500
400
300
13
0
0
14
Fig. 5. Sheet charge conductivity of 2DEG and
2DHG in three modulation δ-doped quantum
wells.
(b)
200
400
600
800
1000
Position (nm)
1
50Å pMD
4 1018 cm-3
40Å nMD
×
1 1020 cm-3
30Å pMD
×
2 1018 cm-3
40Å nMD
Undoped[6]
20
40
60
80
Current Density
(A/cm2)
100
Fig. 6. Gain characteristics of modulationdoped QWs. Both n-doping and p-doping
reduce transparency current density.
6
Peak
Null
96% AlGaAs
80
4
60
2
40
0
20
-2
0
-4
GaAs
0
50
100
150
-6
Distance (nm)
Fig. 4. Quantum barrier design to
isolate electron transport.
5
4
3
Vgate
Channel
0V
-1V
-2V
-3V
-4V
Vgate
μm device
@ RT
0.8
Ibias
Gate
1
5
Vpn
Injector
0.6
2
0.4
1
0.2
0
0
1
2
0
4
3
Bias Current, Ibias (mA)
Fig. 7. DC performance of FICSL. Above
threshold, negatively biasing the gate would
reduce the light output given the same bias
current to the injector-channel junction.
Injector
2. Outer mesa etch
5. Inner mesa etch
3. Wet oxidation
6. Injector layer removal
4. Inner mesa masking
100
L-I-V Characteristics
×
cm-3
100
10
10
Overall sheet charge density (cm-2)
1. Outer mesa masking
×
1020
200
-4
10 12
10
In0.2Ga0.8As/GaAs 80Å QW
Diode Voltage, Vpn ( Volt)
Conductivity Scaling by 2DEG/2DHG
Material Gain ( cm-1)
Overall sheet conductivity (Ω
Ω -1)
-2
n-gate
Vgate = - 0.5V
2
Fig. 3. Structural comparison of oxide-confined VCSELs: (a) top-contacted (b) intra-cavity
contacted (c) laterally injected FICSL with top gate for modulation (this work). Arrows depict
carrier injection flows. Oxide aperture layers are typically placed on the p-side due to the low
spreading characteristics of holes compared to electrons.
10
n-channel
Fig. 2. Band diagrams of a FICSL (a) on-state: electrons and holes are aligned with
a standing wave E2 peak to provide gain. (b) off-state: a negative bias is applied to
the gate, driving holes away from electrons and reducing output power.
iDBR
QWs
p-injector
1
p-type contact
Al2O3
Vgate = 0V
On-state
1
(b)
pDBR
4
Na-Nd (1018 cm-3)
(a)
n-gate
(covered
with SiN)
Gate
Gate layer
Channel layer
Injector layer
10 μm
11.BCB Planarization
7. Injector metallization
10. Channel metallization
8. SiN Passivation
Channel
Gate
12.Gate metallization
13.Backside AR coating
Channel
Channel
9. Via etch
SiN
50 μm
Injector
`
Fig. 8. Process flow for FICSLs. All steps are done with i-line lithography.
978-1-61284-244-8/11/$26.00 ©2011 IEEE
260
Light Output ( mW)
Fig. 1. Schematic of a FICSL. The injector-channel
junction is DC biased, while the gate is the modulation
terminal.
n-channel
Quantum
3
barrier
0
0
r2: inner mesa radius
d: setback distance
(d=r2-r1)
r1: aperture radius
p-type contact
p-injector
Aluminum fraction (%)
n-DBR
Injector
4
Fig. 9. SEM images of a fabricated FICSL
before BCB planarization (upper) and after
the completion of process (lower).