Temperature-dependent investigations of continuous wave and time resolved photoluminescence of the type II heterojunction model system n-InAlAs/n-InP are presented. Surprisingly at room temperature the emission is completely dominated by extremely efficient interface transitions between two-dimensional (2D) holes in InAlAs, which are minority carriers, and electrons from a 2D gas on the InP side of the interface. The intensity decrease between 7 and 300 K is much less than what is usually observed in three-dimensional material. At temperatures larger than 50 K the carrier lifetime starts to drop up to room temperature from 3.8 ns at 6 K to 1.2 ns at 300 K. Both observations are consistently explained by an increasing occupation of higher subband levels of the interface potential well and an activation of new radiative and nonradiative recombination channels including DELTAn not-equal 0 ones with increasing temperature. The decay time of the luminescence from the 1-mum-thick InAlAs layer is short (0.3 ns at T = 6 K). Only a very small part of the carriers optically generated in InAlAs recombine there. The main part diffuses to and is captured by the interface potential well. The structural perfection of the interface is decisive for this highly efficient recombination. Double-crystal x-ray diffraction, Shubnikov-de Haas (SdH), capacitance-voltage, and calorimetric absorption experiments are used to assess the crystallographic and electronic properties of the interface. Observation of a large number of Pendellosung oscillations in the x-ray rocking curves, which can be perfectly modeled using dynamical diffraction theory, unambiguously demonstrate the quality and abruptness of our interfaces. A 2D electron gas at the InP side of the interface through carrier transfer from the unintentionally doped InAlAs is directly observed by angular dependence SdH experiments. The electron and hole subband structure at both sides of the interface is calculated on the basis of a self-consistent solution of the Poisson and Schrodinger equations including band-gap renormalization caused by many particle effects. Experimental confirmation of the subband structure is obtained by second derivative calorimetric absorption spectroscopy.