Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2002-03-12
2004-07-06
Davie, James W. (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
Reexamination Certificate
active
06760354
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to intersubband (ISB) semiconductor light emitters in general and to quantum cascade (QC) semiconductor lasers in particular.
2. Discussion of the Related Art
In the relatively short period of only eight years since ISB light emitters, especially lasers, were first reported in the literature, they have already reached a high level of maturity, which is amply demonstrated by their technological performance and their frequent use in various demanding applications, mostly in trace gas sensing in the mid-infrared wavelength range.
A conventional ISB laser includes a semiconductor waveguide with an active core comprising a stack of alternating unipolar radiative transition (RT) regions and injection/relaxation (I/R) regions. In the RT regions light is generated by electrons undergoing optical, intersubband transitions in coupled quantum wells or short period superlattices. The I/R regions provide electron transport between successive RT regions. While it is customary that all RT regions are essentially identical to one another, and that likewise all I/R regions are essentially identical to one another, it has recently been shown—by using two different, stacked cascades in one waveguide core—that this is not an essential requirement. [See, C. Gmachl et al.,
Appl. Phys. Lett
. Vol. 79, No.5, pp. 572 (2001), which is incorporated herein by reference.]
Another commonly held principle in ISB technology is that the I/R regions must be doped. In fact, early proposals of ISB injection lasers [e.g., R. F. Kazarinov et al.,
Sov. Phys. Semicond.
, Vol. 5, No. 4, p. 207 (1971)] did not include I/R regions and hence could not incorporate extrinsic carriers in the I/R regions. By extrinsic we mean carriers intentionally added to a region of the device by doping. Experiments on such devices failed to produce lasing action. The failure was due primarily to space charge injection, which did not allow the applied electric field to be uniform across the structure. In contrast, the first demonstration of a QC-laser included both I/R regions and extrinsic carriers (e.g., electrons) in the I/R regions. [See, J. Faist et al.,
Science
, Vol. 264, p. 553 (1994), which is incorporated herein by reference.] However, even in the early stages of that work it was understood that—while dopants were seemingly necessary—they did negatively affect some aspects of laser action. First, impurity scattering considerably broadens the gain spectrum, thereby increasing the threshold current density. Second, impurity scattering shortens the non-radiative scattering time of the upper laser level, thus reducing the population inversion. Third, free carrier absorption by the extrinsic carriers increases the waveguide loss in a region of the waveguide with maximum optical intensity, again raising the laser threshold.
BRIEF SUMMARY OF THE INVENTION
We have discovered that doping all of the I/R regions of an ISB light emitter is not an essential requirement for lasing action. In addition, we have found that the overall performance of an ISB laser is enhanced by making the doping levels different in at least two I/R regions that are contiguous with the same RT region. Preferably, the two I/R regions have doping levels that are at least 100 times different from one another. In one embodiment, one I/R region is undoped, whereas the other I/R region is doped. By undoped we mean that the region in question is not intentionally doped; that is, any doping of such a region or layer is relatively low and typically results from residual or background doping in the chamber used to grow the layers of the device.
REFERENCES:
patent: 6476411 (2002-11-01), Ohno et al.
US 6,344,199, 11/2001, Capasso et al. (withdrawn)*
R. F. Kazarinov et al.,Possibility of the Amplification. . . , Sov. Phys. Semic., vol. 5, No. 4, p. 707 (Oct. 1971).
J. Faist et al.,Quantum Cascade Laser, Science, vol. 264, p. 553(Apr. 1994).
C. Gmachl et al.,Quantum cascade lasers with a heterogenous cascade. . . , Appl. Phys. Lett., Vo. 79, No. 5, p. 572 (Jul. 2001).
Capasso Federico
Cho Alfred Yi
Colombelli Rafaelle
Gmachl Claire F.
Mosely Trinesha Shenika
Davie James W.
Lucent Technologies - Inc.
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