Optical semiconductor device

Details Optical semiconductor device Optical semiconductor device

2001-08-27

2004-08-31

Wong, Don (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

C372S043010, C372S044010, C372S046012, C372S049010, C372S049010, C372S049010, C372S050121, C257S018000, C257S096000, C257S015000, C257S017000, C257S022000, C257S092000, C257S184000, C257S013000, C257S190000

Reexamination Certificate

active

06785311

ABSTRACT:

DESCRIPTION
1. Technical Field
This invention relates to an optical semiconductor device and particularly, but not exclusively, to a semiconductor laser device that emits variable radiation in the wavelength range 630 nm to 680 nm. The laser device may be of the edge-emitting or of the surface-emitting type.
2. Background Art
Laser devices or laser diodes (LDs) fabricated in the (Al,Ga,In)P material system which emit visible light in the 630 nm-680 nm wavelength range are becoming increasingly important components of professional and consumer products. For example, it is envisaged that the Digital Video Disc (DVD) system will enjoy a 635 nm-650 nm wavelength LD capable of delivering up to 30 mW output power up to a temperature of 60° C. The next generation of semiconductor lasers will need an even greater maximum power output up to a higher (eg. 70° C.) operating temperature.
By the (Al,Ga,In)P system is meant the family of compounds having the general formula (Al
x
Ga
1-x
)
1-y
In
y
P, where both x and y are between 0 and 1. One particular advantage of this semiconductor system is that it is lattice-matched to a GaAs substrate when the indium mole fraction, y, is equal to 0.48.
A principal limitation of current (Al,Ga,In)P laser diodes is that they are incapable of operating for long periods (or with a sufficiently low threshold current) at the highest specified operating temperature. It is generally believed that this is caused by electron leakage from the active region of the device into the surrounding optical guiding region and subsequently into the p-type cladding region.
One type of laser device is the separate confinement heterostructure laser. The generic structure of a separate confinement laser structure intended to generate light at 630-680 nm will now be described with reference to
FIGS. 1 and 2
.
Curve (a) of
FIG. 1
illustrates the difference between the &Ggr;-conduction band energy of (Al
x
Ga
1-x
)
0.82
In
0.48
P and Ga
0.52
In
0.48
P, as a function of the aluminium mole fraction in the quaternary alloy. Curves (b) and (c) of
FIG. 1
show the difference between the X-conduction band energy and the &Ggr;-valance band energy respectively.
FIG. 1
assumes that the bandgap difference between (Al,Ga)InP and GaInP is split in a ratio of 70:30 between the conduction band offset and the valance band offset.
It will be seen that the minimum energy in the conduction band of (Al,Ga,In)P is a function of the aluminium content. There is a crossover from a &Ggr;-band minimum to an X-band minimum at an aluminium concentration of about 0.55.
The terms &Ggr;-band and X-band as used herein refer to symmetry points in the Brillouin zone and are standard terms in solid state physics, see for example R. A. Smith “Semiconductors”, (Cambridge University Press, 1978). The terms &Ggr;-minimum and X-minimum refer to the minimum energy level of the &Ggr;-band and the X-band, respectively.
FIG. 2
is a schematic band structure of a separate confinement laser structure fabricated in the (Al,Ga,In)P system. It consists of an n-doped (Al
0.7
Ga
0.3
)
0.52
In
0.48
P cladding region
1
, an (Al
0.5
Ga
0.5
)
0.52
In
0.48
P optical guiding region
2
,
4
, a GaInP quantum well active region
3
disposed within the (Al
0.8
Ga
0.5
)
0.52
In
0.48
P optical guiding region, and a p-doped (Al
0.7
Ga
0.2
)
0.52
In
0.48
P cladding regions. Optical transitions giving rise to laser action in the quantum well active region
3
of the laser diode originate from &Ggr;-electrons in the GaInP quantum well active region.
The electron leakage current consists of that fraction of the electrons which have sufficient thermal energy to surmount the potential barrier on the right hand side of
FIG. 2
, and pass into the p-doped cladding region
5
. It will be seen that &Ggr;-electrons are confined in the optical guiding region (waveguide region) by a potential barrier of only around 90 meV at the interface with the p-doped cladding region. This relatively small barrier height allows a significant proportion of electrons to escape. Moreover, holes in the valence band are confined only by a potential barrier of around 50 meV, and this low barrier height also allows significant carrier escape. Furthermore the X-conduction band in the p-cladding region
5
is some 50 meV below the &Ggr;-cladding band in the waveguiding region
2
,
4
, and this allows electrons to escape from the wavelength region
2
,
4
through the X-states in the p-doped cladding regions. Thus, the laser illustrated in
FIG. 2
has a high leakage current, and so has poor performance at high temperatures.
P. M. Smowton et al. in “Applied Physics Letters” Vol. 67, pp. 1265-1267 (1995) show that an important leakage mechanism for electrons can be via the indirect X-valley of the conduction bands in the p-side guiding and cladding regions of a separate confinement hetero-structure laser having two Ga
0.41
In
0.59
P quantum wells separated by a barrier, or set in an optical guiding region of (Al
y
Ga
1-y
)
0.51
In
0.49
P (where y is variously 0.3, 0.4 and 0.5), and clad with (Al
0.7
Ga
0.3
)
0.51
In
0.49
P cladding regions, doped with Zn on the p-side and Si on the n-side. However, no proposals are made for mitigating the problems caused by loss of electrons via this mechanism.
There have been a number of proposals to improve the temperature performance of laser devices fabricated in the (Al,Ga,In)P system.
T. Takagi et al., “IEEE Journal of Quantum Electronics)” Vol. 27, No. 6, 1511 (1991) have proposed introducing a multiple-quantum well barrier in the cladding region.
In UK Patent Application No. 9526631.8, it is proposed that the insertion of a &dgr;-doped p-type layer in the p-doped cladding region of a SCH laser diode will have the effect of increasing the band bending on the p-side of the hetero-junction and thus increase the potential barrier which is presented to thermal leakage of electrons.
G. Hatakoshi et al., “IEEE Journal of Quantum Electronics, Vol. 27, p1476 (1991) have proposed increasing the doping level of the p-doped cladding region in order to increase the potential barrier between the waveguiding region and the p-doped cladding region. UK Patent Application No. 9626644.0 discloses a semiconductor laser which incorporates an electron reflecting layer, to prevent X-electrons escaping into the p-doped cladding region. UK Patent Application No. 9626657.2 discloses the use of electron capture layers to capture X-electrons, and transfer them to a &Ggr;-confined energy level in the active region. However, the affectiveness of these schemes to improve the temperature characteristics of an (Al,Ga,In)P laser device is currently unclear.
The principle of operation of a multiple quantum well barrier (MQB) is to incorporate an MQB in the p-type cladding region of an SCH laser device. The MQB consists of very thin alternating layers of, for example, (In,Ga,)P and (Al,Ga,In)P (for an (Al,Ga,In)P laser). An electron which has sufficient thermal energy to escape from the SCH structure will be quantum mechanically reflected at each of the interfaces of the MQB. If the layer thicknesses are chosen to be &lgr;/4 in thickness, where &lgr; is the electron wavelength, then a band of energies can be engineered at which electrons will be reflected with a probability of 1. Almost unity reflectivity of the electrons can be engineered to exist well above the classical barrier height. Theoretically, a MQB can increase the effective barrier height by up to a factor of 2 compared to the classical barrier height.
K. Kishino et al. “Applied Physics Letters” Vol. 58, pp. 1822-1824 (1991) and H. Hamada et al “Electronics Letters” Vol. 28, pp 1834-1836 (1992) provide evidence to show that the temperature dependence of the threshold current of shot wavelength lasers is improved through the use of such reflectors. However, the effectiveness of the reflectors is usually inferred from LD operating characteristics rather than from a direct measurement of the enhancement of the barrier height. It is difficult to quantify, therefore, just what advantage has accrued from

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