Compound semiconductor structures for optoelectronic devices

Details Compound semiconductor structures for optoelectronic devices Compound semiconductor structures for optoelectronic devices

2001-05-24

2003-05-20

Fahmy, Jr., Wael (Department: 2814)

Active solid-state devices (e.g., transistors, solid-state diode

Incoherent light emitter structure

In combination with or also constituting light responsive...

C257S087000, C257S090000, C257S096000, C257S099000, C257S918000

Reexamination Certificate

active

06566688

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates generally to the field of optoelectronic devices. More specifically, this invention relates to compound semiconductor structures for optoelectronic devices such as light-emitting diodes, photodetectors, edge-emitting lasers, and vertical-cavity surface-emitting lasers.
Semiconductor devices operating at 1.3 &mgr;m and 1.55 &mgr;m are extremely important for fiber-optic communications. Ideally the devices for these applications should operate at a single wavelength, be robust to environmental variations such as temperature, and be inexpensive to produce. To date, most work has concentrated on producing edge-emitting devices using the InP/InGaAsP material system. These devices employ special distributed feedback structures to control the spectral quality of the laser output. However, the yield of these devices can be poor. Furthermore, because these devices are made of smaller bandgap materials grown on InP, they are highly temperature-sensitive and require strict temperature control. Therefore this type of long-wavelength edge-emitting laser is usually very costly.
An alternative device which may allow single wavelength emission is a vertical-cavity surface-emitting laser (VCSEL). In general, VCSELs are light-emitting semiconductor devices comprising two distributed Bragg reflectors (DBRs), between which lies an active region composed of a material emitting the desired wavelength of light. The DBRs act as mirrors, and define a resonant cavity, and the active region acts as an optical gain medium. There may also be spacers between the active region and each DBR used to define a cavity length. The semiconductor mirror structures are usually doped to allow current flow through the active region.
There are problems associated with prior art VCSELs, some of which have been reviewed in U.S. Pat. Nos. 5,719,894 and 5,719,895 to Jewell et al., the disclosures of which are incorporated herein by reference. In general, the production of VCSELs grown using InP/InGaAsP and emitting in the region of 1.3 &mgr;m to 1.55 &mgr;m has been inhibited because of the high thermal sensitivity and poor refractive index properties of the InP/InGaAsP system. In addition, the production of efficient DBRs for InP substrates is difficult and in practice has been found to be very ineffective.
To overcome the production of poor quality mirrors based on InP, one approach has been to use wafer fusion. In this technique, the active region is grown on an InP substrate and the DBRs are grown on gallium arsenide (GaAs). These wafers are then processed and bonded together under high pressure to form a VCSEL. The drawbacks of this method are possible reliability issues because of the complex processing required and the attendant higher manufacturing cost.
To overcome the limitations of InP/InGaAsP, structures based on GaAs substrates have been proposed for vertical cavity devices. The growth of high quality active material on GaAs for 1.3 &mgr;m and 1.55 &mgr;m emission is a problem which has been investigated using a number of different approaches.
A first approach uses InGaAs quantum dots (QDs) grown on GaAs. This approach has produced photoluminescence (PL) at 1.3 &mgr;m, a resonant cavity photodiode operating at 1.27 &mgr;m, and an edge emitting quantum dot (QD) laser operating at 1.3 &mgr;m. A continuous-wave (CW), room temperature (RT), QD-based VCSEL has also been produced, but the lasing wavelength was only 1.15 &mgr;m.
A second approach uses strained GaAsSb quantum wells (QWs). This approach has produced room temperature PL at 1.3 &mgr;m and an edge-emitting laser operating at 1.27 &mgr;m. (The shorter wavelength of this laser can be attributed to gain saturation at the higher current injection levels due to the limited number of defect-free QWs which can be grown.) This approach has also produced PL wavelengths of up to 1.332 &mgr;m using GaAsSb/InGaAs bi-layer QWs, with a type-II band-edge alignment.
A third approach uses a single GaInNAs quantum well. This approach has produced room-temperature pulsed operation at an emission wavelength of 1.18 &mgr;m with a threshold current density of 3.1 kA/cm
2
. A CW edge-emitting laser having a lasing wavelength close to 1.3 &mgr;m has also been produced when the nitrogen content of the QW is increased to 1%. Threshold currents of 108 mA have been achieved for devices with a cavity length of 800 &mgr;m and an active width of 2 &mgr;m.
These approaches all have shortcomings. First, the wavelengths produced are too short for telecommunications purposes. Second, the quantum dot devices rely on long cavities and use highly reflective facet coatings. Third, the GaInNAs approach is limited because the incorporation of nitrogen in InGaAs to form GaInNAs is technologically challenging for a number of reasons. First, there are problems in reliably incorporating more than 1% nitrogen in the active material. Second, a typical precursor used is based on hydrazine (e.g. rocket fuel), and great care must be taken because of the unstable and pyrophoric nature of the compound. Third, it is not clearly understood how the nitrogen is incorporated into the active region. Although some researchers previously thought that a quaternary alloy is formed, it is now generally believed that nitrogen is incorporated as an impurity or defect state. Such states can introduce non-radiative recombination centers which increase in number as the amount of nitrogen incorporated into the material increases. These states may cause local perturbation, or splitting of the conduction band, allowing longer-wavelength emission to be achieved. However, higher nitrogen incorporation generally shortens device lifetime, consistent with the introduction of defects.
Therefore, a need has arisen for improved semiconductor optoelectronic devices that operate at the desired telecommunications wavelengths of 1.3 &mgr;m and 1.55 &mgr;m.
SUMMARY OF THE INVENTION
In accordance with the present invention, a compound semiconductor device is provided that includes a substrate and an active region disposed above the substrate. The active region includes at least two different pseudomorphic layers, the first layer having the form In
x
Ga
1−x
P
y
As
z
Sb
1−y−z
, and the second layer having the form In
q
Ga
1−q
P
r
As
s
Sb
1−r−s
. The first layer includes at least In, Ga, and As, and the second layer includes at least Ga, As, and Sb. “Pseudomorphic” is defined as having a sufficiently low level of misfit dislocations. Each InGaPAsSb layer is pseudomorphic to the substrate. The substrate is preferably GaAs or Al
p
Ga
1−p
As (0<p<1), or of a material having a lattice constant close to or equal to that of GaAs. For the first layer, it is preferable if x is between 0.05 and 0.7, y is between 0 and 0.35, z is between 0.45 and 1, and 1−y−z is between 0 and 0.25. For the second layer, it is preferable if q is between 0 and 0.25 and 1−r−s is between 0.25 and 1.
Preferably, the band structure formed between the first and second layers has a type-II band-edge alignment. Preferably, the peak transition wavelength is greater than 1100 nm.
Preferably, the first layer is a well region for electrons, and the second layer is a barrier region for electrons. Preferably, both layers form quantum wells and may also form a superlattice.
In another embodiment, the active region further includes a third pseudomorphic layer. This third layer has substantially the same composition as the first layer and may be disposed on the second layer. A variation of this embodiment also includes at least one layer-pair between the second and third layers. Each layer-pair has substantially the same composition as the first and second pseudomorphic layers. Another variation includes a fourth pseudomorphic layer disposed on the third layer, the fourth layer having substantially the same composition as that of the second layer. This variation could also have at least one layer-pair between the second and third layers, each layer-pair having sub

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